E
Geographic Information Systems (GIS), mapping and spatial data
Contributors: Lynn Dyson-Bruce, Paul Gilman, Neil Lang, Bruce
Mann and David Wheatley.
| E.1 Geographic information in HERs: maps and GIS | E.2 GIS and spatial databases | E.3 Using GIS in an HER | E.4 Spatial data standards and documentation | E.5 Historic Landscape Characterisation |
HERs comprise both textual records
of historical and archaeological sites (or events), and also a map based record
of their locations. Historically, in paper-based HERs, the textual (attribute)
record was stored on a card index or record sheet, while the locations
(geographic information) were marked on a series of related maps. HERs in
England usually hold paper maps showing the location of archaeological sites
but while computerised databases have now largely replaced card indexes for
handling attribute data, it has not been until relatively recently that
Geographic Information Systems (GIS) have begun to replicate and enhance the
geographic component of HER data.
E.1 Geographic information in
HERs:
maps and GIS
E.1.1 Mapping
The way in which archaeological
sites have been mapped varies, but two main types of mapping can be found, with
some HERs maintaining both types (Baker 1999a):
Archaeological constraint areas: this type of mapping attempts to show areas within which
archaeological remains are known and/or suspected to survive. This helps archaeologists, planners,
developers and others to see rapidly whether a particular development is likely
to require an archaeological response.
This may also mean that the areas outside the constraint line might be
considered to be devoid of archaeological interest and some HERs have, for this
reason, decided not to define areas of constraint.
Archaeological extent: in this type of mapping, the areas indicated are the extents
of known archaeological monuments.
Where this extent is only approximately known then it is normal practice
for a line to be defined around the nearest field boundary. It is usual for linear features such as
Roman roads to be delineated and also for find spots to be depicted by a spot
or a circle. Where there is uncertainty
as to whether the location is correct, a dashed line may be used to indicate
this.
The scales used by HERs will vary
but the basic mapping is usually 1:10,000 with more complex areas such as
historic towns mapped at 1:2,500 or even 1:1,250. Some HERs have mapped directly on to paper or film copies of the
OS maps, whereas others use overlays.
The advantage of the latter approach is that the overlay is independent
of the OS map base, which changes over time.
In practice, most HERs will have a
variety of types of maps that have been developed for different purposes and projects. For example, crop-mark sites may be plotted
on separate overlays, which can be placed over the main HER maps to enable the
user to see the crop-mark features within each HER monument.
E.1.2 Mapping issues
In HER mapping it is important to
consider the following:
·
the
HER recording manual and user guide should explain how sites have been mapped.
·
a
consistent approach should be taken to mapping, especially across each type of
mapping.
·
there
should be consistent use of symbols, which should be defined in a key either on
the map itself or in the HER recording guidelines or user guide.
Maps form a fundamental tool without which HERs would be unable to function. However, paper maps have their limitations. It can be difficult to keep the map base itself up-to-date. Maps can be time-consuming to use and can be viewed by only a few people at any one time. Only a limited amount of information can be shown on one set of maps or overlays, making it harder to carry out assessment and analysis. For these reasons, most HERs have adopted or are exploring the use of GIS.
E.1.3 GIS
and spatial databases
GIS has much to offer within local
government, and is now established as the tool of choice not only for managing
spatial information both within archaeological and heritage contexts, but also
for all other areas that deal with spatial data including environmental
management, planning, rights of way, flood management and many other topics.
As a result, many local authorities
have or are establishing corporate GIS-based databases helping to avoid
duplication of effort, make best use of resources and bring together datasets
which were becoming fragmented. Linking
an HER dataset into a corporate GIS means that the HER data can be displayed
and related to other datasets held in the authority. These may be topographical, such as contours and rivers, or other
planning constraints such as conservation areas and SSSIs. This opens up new possibilities for taking a
more integrated approach to planning and conservation.
GIS also opens up avenues for
analysis and research into the historic environment. As desktop GIS software develops and its power continues to grow,
there is increased potential for analysis and visualisation of datasets, for
example in three dimensions or in virtual-reality models. Recent development of web browsers
incorporating GIS is enhancing the potential for sharing and display of
information through corporate intranets and the internet.
There are now many books on the uses
of GIS in archaeology, including edited volumes illustrating the uses of GIS
for research and management of projects (see for example Gillings et al 1999, Lock 2000, Westcott and Brandon
2000) and also more general sources (Wheatley and Gillings 2002, Connolly and
Lake in press). The ADS's GIS Guide to Good Practice (Gillings and Wise 1998) provides
practical guidance for individuals and organisations involved in the creation,
maintenance, use and long-term preservation of GIS-based digital resources and
also provides specific advice for HERs.
The aim of this section of the
manual is to provide a primer on some of the issues to consider before
embarking on system development. It
does not set out to review general functionality of GIS in any detail or to review
the current GIS market place. Rather,
it sets out some of the considerations to be taken into account in establishing
a GIS for an HER, and some of the benefits that can be gained through
successful implementation.
E.1.4 Having a GIS strategy
Desktop GIS applications are
relatively affordable, and run on commonly available PC platforms. However, the overall systems cost to put
together an HER application (including procurement, customisation, data
capture, maintenance, mapping) remains high.
Although elements of the system-development life cycle may sometimes be
missed out, this will generally be a false economy. For example, omitting a clear and well-thought-out statement of
user requirements will, at the minimum, make it difficult to measure whether
the system has fulfilled its anticipated purposes, and at the worst, result in
frustrated users and managers who feel ‘let down’. The development of systems that will meet needs over the life of
the system and that enable the information held within them to be transferred
to future systems requires careful planning.
Significant changes after system implementation can be very expensive.
Many local authorities will have
either departmental or corporate policies governing GIS. These may include standards for hardware and
software, data standards and policies for access. GIS is well suited to a corporate approach to data management,
since it can bring together information from different sources, and even
different data types into a single, spatial view. For example, GIS allows users to select a location (for example a
property address) and to display text information from a database of planning
proposals, a listed buildings database, an HER database or other digital
information such as a scanned property deed from the record office or
photographs from the engineer's department.
As with most computing, the
continuing emphasis on communications and IT in higher education ensures that
there is a growing awareness of GIS amongst recent graduates. For existing HER staff, training in the
corporate GIS is generally available either from the local authority or from
commercial training providers. There
are also courses offered by university continuing education departments and
others on the use of GIS in archaeology and for conservation.
E.2
GIS and spatial databases
| E.2.1 Modelling and documenting spatial data | E.2.2 Systems to work with spatial data (GIS) | E.2.3 Sources of spatial data | E.2.4 Precision and accuracy | E.2.5 Useful websites and references |
Geographic Information Systems are
conventionally defined as systems that capture, store, manipulate and output
geographical information. Geographical
information may be considered as information that is tied into some specific
set of locations on the earth’s surface, including those immediately adjacent:
the sub-surface, oceans and atmosphere.
‘Spatial’ is now starting to become a commonly used term or synonym for
‘geographical’. The term ‘geospatial’
is gaining currency and also describes the trend towards convergence of spatial
technologies such as Global Positioning Systems (GPS), aerial and remote
sensing and context-aware computing.
GIS emerged from three principal
roots: the need for data analysis and display tools, the automation of map
production, and landscape architecture and environmentally sensitive planning.
Although GIS have been available since the 1960s, it is only in recent years
that hardware and software have become sufficiently powerful and inexpensive
for its use to become widespread.
E.2.1 Modelling and documenting
spatial data
Layers (themes)
Spatial databases are structured into thematic layers. This means that the system stores geographic data according to the real-world theme to which it relates. A typical spatial database may have separate layers (themes) to represent:
·
Topographic
height (for example a digital elevation model)
·
Soil
type
·
Rivers
and streams
·
Roads
·
Archaeological
monuments
Vector and raster data
There are two main types of data
used in GIS layers: vector and raster, which differ principally in how the
system represents the geographic component of the data in a computer file. In a
vector representation, the spatial database contains a geometric description of the theme in question,
while in a raster representation regularly spaced samples of something are stored.
Vector data is therefore similar to
data in a CAD package. Each element in the layer is represented by some
geometric entity such as a point, line, or polygon. The process of creating
vector data is similar to drawing, using either a digitising tablet or by
drawing objects on the computer screen and can be time consuming and expensive.
This has the advantages of providing a compact data-storage format, allowing
scalable presentation. Being based on geometric objects, it is straightforward
to link these to text-based records. Vector representation permits easy
quantification of areas and some analytical methods such as network analysis.
Ordnance Survey Landline mapping captured at base scales of 1:1,250, 1:2,500
and 1:10,000 for urban, rural and upland/moorland areas respectively, is an
example of vector data (in this case containing many different layers). Increasingly though OS Landline mapping is
being replaced by the topography layer of OS MasterMap®.
Raster layers are more similar to
digital images, as they are made up of a grid of cells, each of which contains
a value at a particular location. Raster data is usually generated
automatically by scanning paper originals, or obtained from digital sensors (in
cameras, or satellites) and is therefore often rapid and cheap to generate.
However the quality of the raster dataset is dependant on both the resolution
the image is captured at and on the qualtiy of the locational data fixing the
position of the image within the GIS. This is particularly suitable for
applications requiring display of fine detail (for example aerial photography
or historical mapping) but also facilitates many forms of terrain analysis and
simulation modelling. Raster data may be dichotomous (that is, cell values are
either 1 or 0, to provide a black and white picture) or continuous, where each
cell may be assigned a range of values. Examples of widely-used raster data
sources include the OS Siteplan® data (for scales between 1:500 and 1:2,500) and Ordnance Survey
1:10,000 and 1:25,000 scale raster products. Raster data is almost always
supplied in pre-defined areal units or ‘tiles’ based on the OS National Grid.
Typical vector applications include spatially
referenced database applications – for example, location maps, sites,
monuments, artefacts – mapping
applications, managing networks (such as roads and utilities) and terrain
analysis using TIN elevation models. Raster themes are often employed to
analyse continuously varying layers such as slope, elevation or resistivity and
remote-sensed data such as satellite imagery.
Analyses that employ raster data typically include neighbourhood
analysis and overlay operations (for example reclassifying two separate maps of
land use and height to obtain an intersected model of land use at height),
simulation modelling, predictive modelling, decision support, cost surface and
optimum route analysis and visibility analysis.
Many themes could be represented by
either vector or raster data models. Terrain, for example, can be represented
either by a vector model, using a network of triangles (referred to as a
Triangulated Irregular Network or TIN), or by a raster altitude matrix in which
each cell contains the elevation at that location. The choice of representation
depends on a range of factors, including the capabilities of the software,
availability of source data and the intended uses of the data.
Fortunately, most major GIS now work
with both types of information, and can use them effectively together. Many forms of analysis (such as visibility
analysis or hydrological modelling) can be undertaken using either raster or
vector layers, and the two can also be employed together as, for example, when
a satellite image is ‘draped’ over a vector terrain model, creating a ‘digital
landscape’ which a user can explore (rather like a virtual reality ‘fly
through’ (see Figure 39)). It is also possible to
automatically convert data from vector to raster and – with some limitations –
from raster to vector when needed.
Figure 39: Using three dimensional
modelling in GIS to examine sites in their landscape setting. [© Essex County Council 2007 and © Crown Copyright, All rights reserved. 100019602. 2007]
It is important to note that users of third-party data should be aware of how the data was created, if good control of spatial accuracy is to be obtained. Ideally, spatial data should be supplied with metadata that records how and when the data was captured, and how it was georeferenced to the National Grid. This is particularly relevant with third-party surveys, where it is vital to fully record and understand the precision and accuracy of the survey and the methods used to georeference it. The widespread use of Global Positioning Systems (GPS) to undertake new surveys has recently made this even more pertinent (see below).
Attribute, spatial and
topological components
The choice of vector or raster representation is not the only factor in the design of a spatial database. A fully geographic database will consist of three separate information aspects: attribute, spatial and topological components.
The attribute component refers to the non-geographic content of the
data. In an archaeological context, this will include any observations about
interventions, condition, sources and so forth that are stored in a
conventional database.
The spatial component refers to the location of the site or event in
geographical space. Not all attributes may have a geographical component, and
where they do this may be represented by either vector or raster models.
Spatial data, however, also requires
a topological component in order to
fully describe the connections between geographic entities. This contains, for
example, the intersections between lines of a network (such as rivers) and the
geometric relationships between polygons in a vector theme.
Most large GIS systems will provide
mechanisms to store and manage all three of these either ‘in-house’ or by
allowing links to be made with external data sources, as where a polygon on a
GIS layer is linked to an event record in a conventional database.
Georeferencing, projections
and datums
The term ‘georeferenced’ describes data whose position in geographical space is fully recorded, usually in the form of standard cartographic grid references. The process of fully recording the geographic location of a data theme is usually called georeferencing the data, and is one of the most important stages in the creation of a geographic database.
Georeferencing usually comprises two
steps. Initially, the data will be digitised in whichever system of coordinates
is used in the original map. For vector data, this will involve calibrating the
digitising device to the coordinate system on the map (it is also important to
try to estimate and record the level of precision to which the map is digitised,
as this contributes to the accuracy of the produced data). For scanned data,
the initial stage of georeferencing involves at a minimum locating the corners
of the raster grid in geographic coordinate space. In many cases, such as
aerial photographs, this is not sufficient to accurately georeference all the
raster cells in the theme and it is necessary to set up a more complex
coordinate transformation between the raster and the coordinate system, usually
by identifying control points on the raster and entering their known coordinate
positions. In the UK the coordinate system used for georeferencing is usually
the OS National Grid (more properlly called OSGB36, see below) and if all data
is recorded within this sytem, it may not be necessary to delve further into
the complexities of georeferencing.
To integrate OSGB36 data themes with
sources of data referenced to other coordinate systems, however, it is necessary
for the spatial database to also have a full description of both coordinate
systems. This is required, for example, for the integration of digitised map
data with independently recorded GPS readings. In this case, features on the
map will ‘know’ their OS National Grid locations, but the GPS readings may be
in a different GPS coordinate system (for example WGS84). Unless the spatial
database contains information about how each of these coordinate systems relate
to a common reference system, it will not be possible to visualise or analyse
these two data themes together. This usually involves recording at least the
projection and the horizontal datum of the coordinate system that is used. This
should be printed on the maps, or can be found in publications of the agency
who defined or maintain the coordinate system.
It is usually this second part of
georeferencing which leads to confusion, although fortunately many contemporary
GIS are now supplied with a wide range of pre-configured coordinate systems,
projections and datums which can make things much easier. In order to fully
understand georeferencing, however, it is advisable to have some basic
understanding of geodesy: the study of the shape of the earth and the
determination of the exact position of geographical points. This is
increasingly important because of the growth of surveys undertaken with GPS
receivers and is particularly problematic when it comes to integrating data
about topographic height.
Coordinate systems in the
UK
The traditional coordinate system,
on which most of the UKs mapping is based, is usually called the National Grid.
More fully, it is the Ordnance Survey of Great Britain 1936 (OSGB36) and has
historically been defined by a a national network of triangulation pillars. It
is based on a Transverse Mercator projection, made about the 2 degree west
meridian, with the crossing point of this and the 49 north parallel defining a
false origin of 400,000 N, 100,000 E. Height values may be based on one of
several vertical datums around the UK, but for mainland Britain they relate to
the mean sea level at Newlyn in Cornwall (or, more properly, on observations of
those made between 1915 and 1921).
The widespread availability of
accurate, globally-reference survey coordinates has recently rendered
traditional triangulation networks effectively obsolete, and the National Grid
is no longer defined in those terms. Ordnance Survey have therefore established
a new national positioning infrastructure based on the European Terrestrial
Reference framework (ETRS89). This is maintained by a group of permanently
installed GPS receivers around Great Britain, referred to as Active Stations.
These coordinates are now used to define OSGB36 through specifying how to
convert between ETRS89 coordinates and OSGB36. The OS provides transformations
for both plan position (National Grid Transformation, or OSTN02) and for height
values (National Geoid Model, the OSGM02).
Collecting accurate GPS data in the
UK that can be accurately positioned on existing data usually now involves (i)
establishing the receiver’s position in the ETRS89 system and then (ii)
converting those surveyed coordinates using OSNT02 and OSGM02 to their
equivalent OSGB36 coordinates. These stages may be undertaken either at the
time of survey or later, and may be done within the GPS receiver itself, or in
post-processing software.
Older or simpler GPS data may
provide coordinate values in the World Geodetic System (WGS84). This differs
from ETRS89 in that WGS84 is not tied to any point on the earth’s surface. As
such, it can be problematic to accurately convert coordinates in WGS84 and
similar global systems to map coordinates because the surface of the earth is
not stationary (plate tectonics can move two positions on the earth’s surface
by as much as 2cm in one year).
An excellent introduction to the
issues arising from use of GPS-derived coordinates, from an archaeological
perspective, can be found in Ainsworth and Thomason (2003).
OS National Grid
The OS National Grid is one of several possible means of describing a position in space, although it is probably the most widely used referencing system in archaeology. The National Grid for Great Britain is a descriptive grid drawn from a single false origin (lying to the south-west of the Isles of Scilly) that allows for points to be referenced to a notional sub-metre accuracy. Conventionally map sheet letters preface points on the grid, for example SK 12345678, where the map sheet represents a 100-kilometre square (SK).
The National Grid also allows for
absolute references expressed in a fully numeric format. For example the reference 345678987654 refers
to a location 345678m east 987654m north of the origin. This numerical format
for references is convenient as most GIS packages do not recognise the letters
associated with map sheets, using co-ordinate systems that depend entirely upon
numeric fields.
Ideally, a location will be
identified to the nearest metre within the National Grid. However, it is not
uncommon to use less precise references – to the nearest 10 or 100m – by
omitting the least significant digits. For example, a reference with only six
figures after the letter code (for example SK123568) or with only 10 digits
(3456898765) refers to a location with a 10m precision, while SK1257 or
34579877 have 100m precision. Clearly it is vital to treat locations supplied
with 10 or 100m precision references with care: they frequently need to be
interpreted to mean ‘the location is somewhere in the square whose origin is
specified by the grid reference’ as opposed to “the location is within 10 or
100m of the reference”. For this reason both the original (source) formats of
coordinates should be stored in HERs, as well as any ‘GIS friendly’ derived
coordinate values (see also section E.2.4).
Note that, although most absolute
grid references will be expressed in twelve figures, all sites locations in OS
100km map squares commencing with H (All of Shetland and much of Orkney) will
have seven figure northings whilst the following map squares (NA, NF and NL
(all covering the Western Isles) and SV covering the Isles of Scilly, have only
five figure eastings.
An online tutorial on the National Grid can be found in the education section of the OS website http://www.ordnancesurvey.co.uk/oswebsite/gi/nationalgrid/nghelp1.html
E.2.2 Systems to work with spatial
data (GIS)
GIS software
There is a trend towards GIS
becoming easier to use (for example through familiar Windows-type Graphical
User Interfaces or GUIs), with a diversification in the market place to
distinguish between the needs of 'heavyweight' applications, and day-to-day
data analysis and visualisation (sometimes characterised as ‘doers’, ‘users’
and ‘viewers’). Modern desktop GIS applications software (such as Maplnfo®,
ArcView®, ArcGIS®, GeoMedia®) are powerful
programs in their own right, however related software products (ER Mapper,
ERDAS® Imagine) combine sophisticated tools for image-processing
techniques, such as orthorectification and mosaicing aerial photographs, with
some GIS functionality and can be used to prepare raster layers prior to
importing them into a GIS. GIS functions are also being incorporated into a
number of other software products ranging from AutoCAD® – where a
fully-functioning GIS has been built ‘on top’ of the CAD system – down to the
provision of basic mapping functions in Microsoft® Excel®.
Procuring GIS software
If a local authority has chosen a particular GIS, this may be a powerful argument for the HER to do likewise. A corporate GIS will make the task of sharing data with other sections easier and will enable the HER to harness the expertise within the authority, helping to support the system, and possibly to obtain the software at low or no cost. HER managers should still check that this software meets their requirements. These requirements must be realistic - think about how much a facility would be used, and if the requirement is occasional, whether there are cheaper ways of meeting the need, such as using an external contractor.
One element of the user requirement
is likely to be a list of the functions that the GIS is intended to
perform. A useful source of advice is
the Functional Requirement Specification
for GIS (LGMB 1991), available from the Improvement and Development Agency,
formerly the Local Government Management Board (LGMB). This includes a catalogue of GIS functions,
which can be used as a 'checklist' to compare different software products and
to assess if any customisation might be required and what skills would be
needed to achieve the desired outcome.
Target response times for operations that are important to users can
provide a useful benchmark and can be used to make sure that the users'
expectations and the developer's system performance targets are aligned. For example, if the identification of all
records falling within an administrative boundary will be a frequent enquiry
what would be the maximum acceptable time for this to take?
E.2.3 Sources of spatial data
Spatial data from paper
mapping
Where paper mapping exists and there
is a need to generate GIS themes from it, then there are essentially two
choices: scanning (which produces raster data) and digitising (which produces
vector data). The choice of these will partly be constrained by cost – it is
far cheaper to scan than to digitise – but this needs to be balanced against
issues of storage and usefulness. As a generalisation, scanned data is useful
for visualisation and for making maps, and is increasingly useful as a
preliminary step in digitising (see ‘heads up digitising’ below). Vector data,
on the other hand, is far more flexible where analytical use is foreseen or
where output mapping needs to have different components of the input data.
Scanning can be undertaken in-house,
although scanners large enough to process whole maps are expensive
(approximately £3,000 - £15,000 depending on features) and time consuming to
set up and use. More commonly, the scanning of a document archive will be placed
in the hands of an outside agency and it is therefore vital that a clear job
specification is established in advance. This should cover the resolution of
the scans, whether they will be monochrome, greyscale or colour, the file
format and compression to be used and should also make clear what quality
checks will be undertaken and whether the job includes user-intervention to
clean the data after scanning. Agencies will probably supply scanned data on
CD-ROM or DVD-ROM and it is advisable to make archive copies of these original
materials quickly and store these under archive conditions.
Digitising paper sources is far more
time consuming, as it involves attaching the map to a digitising tablet and
tracing over the different data elements to, effectively, ‘draw’ the require
data manually. To some extent, digitising in this way has been overtaken by the
use of ‘heads up digitising’ (see below) and by automated raster-to-vector
tools but it remains true that the creation of accurate, well-structured, topologically
correct vector data requires considerable level of human intervention and is an
order of magnitude more time consuming (and hence expensive) than scanning. The
pay-off for this effort, however, is that the resulting data is a great deal
more structured and useful for analytical purposes than any raster product. As
with scanning, it is possible to undertake digitising ‘in-house’ using
digitising tablets. These are available in sizes from A5 up to A0 although
larger tablets are expensive to purchase and difficult to support. The vast
majority of HERs are unlikely to be able to justify purchase of this kind of
equipment unless it is as part of a wider institutional project supporting
other areas as well as archaeology. As such, digitisation projects are likely
to be undertaken by outside contractors as with scanning and it is even more
vital that a thorough agreement is made in advance covering the accuracy,
precision, quality and format of supplied data.
‘Heads up’ digitising refers to the
two-stage process whereby maps are initially scanned, and then vector data is
traced from the scans using the computer screen rather than the digitiser.
Increases in computer power and screen resolutions in recent years have made
this an attractive method of digitising data, which can often be undertaken by
users themselves on an ‘as needed’ basis (although it is difficult to establish
good quality standards in this way). This also makes it more attractive for
HERs to scan paper archives because, if collected with the possibility of
heads-up digitising in mind, then this is now an open-ended strategy that does
not preclude generation of vectors in the future.
Moving between vector and raster
data is possible in both directions, although it is important to understand the
limitations of this. It is relatively straightforward to generate raster data
themes from vector data (for display or for analysis in raster-based GIS) and
most commercial GIS will provide functions for this. Generating vector data
from raster (scanned) themes is, however, far more difficult as the computer
needs to make ‘intelligent’ choices about how the pixels in the raster relate
to geometric entities (lines, areas and so forth) in a vector theme.
Nevertheless, software is available which will take scanned maps, such as
contour maps, and generate vector outputs in the same way that Optical
Character Recognition (OCR) software can generate text from scanned documents.
These can be a valuable first step in generating vector data, reducing some of
the tedious line-following procedures, but (as with OCR software) it is
important to realise that none of these are foolproof, and that automatically
generated vector themes will still require considerable user intervention to
produce topologically-complete, clean vector data. There is a strong argument
for HERs to refrain from becoming involved in decisions about how vector data
is generated, but rather to concentrate on setting down the specification (in
terms of quality, accuracy, precision and so forth) of the data that is
required, and then leaving it to agencies to decide if that is best delivered
by full automation, partial automation or human intervention.
Commercial mapping
One of the principal sources of
commercial mapping in the UK is the OS, which now provides a wide range of
mapping and other spatial data in digital formats. These may be available to
HER maintainers as part of wider corporate licensing arrangements. All OS data
is subject to Crown Copyright and arrangements will exist in most organisations
for appropriate copyright statements to be included with any maps or output
that uses OS data.
Ordnance Survey provide both raster
products and also vector data sets. Raster products are essentially scanned
versions of OS maps available at a variety of scales and useful as backdrops
for creating, for example, constraint maps. Raster data is available at
1:250,000, 1:50,000 and 1:25,000 scales for different mapping purposes.
Ordance Survey vector datasets are
also available at a variety of spatial scales, and with a wide variety of
thematic information. Among the more useful of these are the Landform PROFILE®
dataset, which represents contours derived from 1:10,000 scale mapping and the
LandLine® dataset which contains layers representing a wide range of
manmade environment including houses, factories, roads, and administrative
boundaries as well as heritage features. LandLine® data is scaled
according to the region with 1:1,250 scale data for urban areas; 1:2,500 scale
in rural areas; and 1:10,000 scale for remote areas such as mountains and
moorland.
The Ordnance Survey’s digital
datasets (particularly LandLine) are currently being replaced with a new
delivery mechanism for GIS data called OS MasterMap® (see below).
Other commercial digital map sources
area available, although none can compete for completeness or up-to-date survey
with the Ordnance Survey. However, for some tasks where OS data is not
available, it may be possible to turn to providers such as Bartholemews (http://www.bartholomewmaps.com/ )
for particular digital map data supplies.
One of the most widely used sources
for digital data is aerial photography, which may be held by larger HER
maintainers in the form of negatives or prints. These can be scanned and
georectified to provide not only data relating to crop and soil mark sites, but
also additional map detail that may not be available in commercial mapping.
Aerial photographic coverage can also be purchased commercially or licensed from
the Ordnance Survey or from a range of commercial resellers such as Getmapping®
(http://www.getmapping.co.uk/).
OS MasterMap®
OS MasterMap® is an
intelligent digital map designed by the OS for use with geographical
information systems (GIS) and databases. It provides significant advantages to
users of digital data, including seamless delivery, the use of a new coding
system and improved metadata about spatial entities. However, it requires
significant investment in new software in order to use it.
OS MasterMap® comprises
topography, imagery, address and ITN (integrated transport network) layers.
Based on the National Grid, the topographic layer contains information on every
landscape feature – including buildings, roads, archaeological features - and
represents a significant evolution from traditional cartography. MasterMap®
depicts the real-world digitally and presents this information as themes in a
series of layers, each layer carrying millions of features. Each feature has
its own unique identifier or TOID® - a 16-digit reference number
that can be shared with other users across different applications and systems.
This allows easier data association and greater accuracy, focusing on
real-world objects on the map. In addition, the Ordnance Survey have released a
high quality Imagery Layer whose images have been fully orthorectified to
represent truly and accurately what is on the ground. The Imagery Layer is available at 25 cm resolution and 24 bit colour.
The Address Layer of OS MasterMap® provides precise coordinates for
more than 26 million residential and commercial properties in Great Britain,
whilst the ITN layer is probably of less use to the HER officer. It enables business needs from navigation to
asset management and from traffic analysis to accessibility studies.
Users of MasterMap® data
should also be aware that, in addition to the data format itself, the delivery
of data with improved positional accuracy (see Positional Accuracy Improvement
programme below) differs between MasterMap® and older formats and
digital products such as Landline®, Profile® and Panorama®.
Further details can be obtained from
the OS website http://www.ordnancesurvey.co.uk/oswebsite/products/osmastermap/
OS
Positional Accuracy Improvement (PAI) Programme
Since April 2001, the Ordnance
Survey have been undertaking a Positional Accuracy Improvement (PAI) programme
of the 1:2,500 small town and rural map base throughout Great Britain. PAI captures data to a greater absolute
accuracy in relation to the National Grid, resulting in improved and more
consistent data. It is claimed that PAI
will also future-proof the data for the addition of new feature detail as well
as providing a better relationship between the OS 1:2,500 map data and
information captured through GPS.
In simple terms, data that has been
through PAI will have better absolute and relative accuracy than previously
supplied data, but at the expense that it may no longer match existing (legacy)
data products. Where HERs contain data that has been created by reference to OS
data products, there is now a possibility that this data will appear to be in
error because the underlying OS mapping has ‘moved’ slightly. This movement
should not be excessive, but may in some cases result in changes of up to a few
metres.
It would therefore represent good
practice for HER officers to undertake an audit of the data they are
responsible for maintaining to assess how that data was originally created. The
spatial elements of most databases were created against 1:10,000 paper maps
with grid references expressed to the nearest 6 or 8 figures (that is to
nearest 100m or 10m) and should not be affected by PAI. However, if the
boundaries of an HER record are, for example, delineated along the
representation of field boundaries on a 1:2,500 tile, then the shape of that
land parcel may be altered through the transformation processes resulting in
errors in the HER data. Those errors may include spatial (position) errors, but
may also potentially result in topological errors (such as ‘slivers’ and
‘gaps’) where data is processed against new OS datasets.
Where migration to post-PAI Ordnance
Survey data is necessary, then a variety of assistance and tools are available
from OS and from third-party GIS manufacturers to help update HER data to match
OS mapping. These include ‘link files’ of coordinate corrections and processing
tools which, in combination, can provide for automatic (or semi-automatic)
updating of spatial data themes according to the known changes in spatial
position.
Ordnance Survey have produced a
series of documents including a “PAI Companion” that explains the workings and
implications of the PAI in more detail. This can be downloaded in PDF format
from the OS website, which also contains the most up-to-date information about
the progress of PAI (see http://www.ordnancesurvey.co.uk/oswebsite/pai/).
E.2.4 Precision and accuracy
Precision and accuracy are important
issues in GIS. Precision is the degree
to which a measurement is refined while accuracy records the correctness with
which the measurement is taken (Richards and Ryan 1985). Thus a six-figure grid reference may be
accurate but imprecise while a twelve-figure grid reference may be precise but
inaccurate.
Precision and accuracy become
significant when comparing disparate datasets is made possible with GIS. These sorts of issues come to the fore in
GIS because vector displays can give a spurious impression of highly precise
and accurate mapping. Scale, the ratio
between distances on the map and in real space, can be manipulated almost
infinitely in a vector GIS, as areas are
‘zoomed' or 'panned' to suit.
However, simply because it is possible to zoom does not mean that the
data thus displayed will be accurate at the new scale. OS contour data, for example, may be
digitised from 1:50,000 map sheets, at which scale the smallest distance that
can be distinguished is 0.5mm or 5m on the ground. Because this data can be reproduced in the GIS at 1:10,000, at a
resolution one fifth of the original, it does not become more accurate. Thus a point captured on screen against a
1:50,000 map base will be accurate at that scale and not progressively more
accurate to any larger scale to which the map has been zoomed.
There are two approaches to
representing the precision with which heritage objects are located within the
GIS. The first approach (Figure 40), adopted by the former Archaeology Division
of the OS, places the object in the bottom south-west corner of a virtual
square somewhere in which the object is located. For example, an object recorded as a four-figure reference (such
as TQ 77 89) could lie anywhere within that 1 kilometre square. Similarly, a six-figure reference (such as
TQ 724 876) could lie anywhere within a 100 metre square. In both cases, the object would be
represented as a point marked on a map in the south-west corner of the
appropriate square, that is, the point marking TQ 77 89 would be marked at TQ
7700 8900. Variations in this approach
include placing the point in the centre of a square rather than the south-west
corner, (that is, the point marking TQ 77 89 would be marked at TQ 7750 8950)
or in the centre of a virtual circle.
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Figure 40: Representing the location of a heritage object within a 'virtual
space'.
This approach has the advantage
that, since most four-figure references are for stray finds, representation as
a point bears some relation to the object depicted. The approach has the disadvantage that the object will only be
retrieved by a spatial search that includes the point (whether located in the
south-west corner or centre) although the implied imprecision means that the
object could derive from a wider area.
The second approach (Figure 41) attempts to overcome the spatial retrieval problem by representing the object as the 'physical space' within which the object might lie, so that a square or a