Home Articles Basic concepts of spatial and Geo-Referencing accuracy

Basic concepts of spatial and Geo-Referencing accuracy

Prof Prabhakar Misra
Prof Prabhakar Misra
Director, GIS Institute
[email protected]

The concepts of the accuracy regarding plan position of topographical features and their elevations (heights) are of paramount importance to a Geographic Information Systems (GIS) professional as well as a map-user. These concepts have gained added importance because of the GIS in which all the layers pertaining to the different themes (e.g. soil, geology, water resources etc. are ‘tied down’  to the layer of topography which, inter alia shows buildings, road networks, streams etc. It is this topography-layer which acts as a base layer and provides the positional accuracy and much desired geo-reference (connection to lat/long or any other coordinate system) to all the spatial features in the GIS database.

If the topographic base layer has not been prepared keeping the positional accuracy in mind, the whole GIS will behave like a floating system. On close scrutiny,  we find that some GIS projects have indeed turned into F GIS (Floating GIS) projects because of the wrong geo-referencing. Some of the considerations which go in designing the requisite accuracy are discussed further. These are not exhaustive but should be able to provide adequate guiding frame-work for accuracy towards designing a GIS project, specially, the topographic layer.
Table 1: Size of the ground for the point at various sacles

Scale Dimension in Terms of Ground
1:10,000 0.2 x 10000mm  
  (0.2 x 10,000cms)/10 ………………… = 200cms
  (0.2 x 10, 10,000 Metres)/10 x 100 ……… = 2 Metres
1:5000 ……………………………………………………. 1 Metre
1:4000 ……………………………………………………. 80 cms
1:2000 …………………………………………………. 40 cms
  (0.4) Metre
1:25,000 …………………………………………………. 5 Metres
1:50,000 …………………………………………………. 10 Metres

Accuracy of a Topographical Map-Point
It is assumed that a point on a map will have a minimum size of 0.2 mm. In order to convert this dimension in terms of the ground, it is to be multiplied by the scale of the map. For example, Table 1 shows the size on the ground for the ‘point’ on various scales.

The table 1 clearly indicates that if the map is on a scale of 1:25,000, one will not be able to get better measurements than 5 metres.  Thus 5 metres is the size of the smallest object on 1:25,000 scale.

Accuracy of a line


Imagine that we have to make a linear measurement between two points A and B on a map to determine the length of a highway, length of optical fibre cable or plan a housing complex. The measurement is going to be affected by the propagation of inaccuracies of both the points.

In statistical terms, the accuracy of the linear measurement is taken as Ö2 x accuracy of point
1. 41 x accuracy of the point. 

Example
Linear accuracy on 1:10,000 scale (Ö2 x2)= 1.41 x 2 metres = 2.82 metres say 3 metres.
On 1: 2000 ( 1.41 x 0.4) = 0.564 = 0.6 metres
What it suggests in practical terms is that if the requirements of accuracy can be met by a smaller scale (1:10,000) one need not demand a larger scale is of 1:2000. The effort in producing a map on scale of 1:2000 are not five times the effort needed for 1:10,000 map but may be 7 to 8 times. Besides, more time and better equipment may be required to do the job thereby increasing the cost.

Here the stress should be on the Need dictated by the accuracy considerations and not the DEMAND for a working space. We have other solutions for this requirement of working space/paper space.

Accuracy of Heights on Maps
Many map – users would not have noticed the legend box printed on the lower right hand corner of a printed Survey of India Map. The legend gives the following category of heights.

  • Heights : triangulated; station point, approximate, shown as under
    Heights, triangulated: station point; approximate
    200               .200                       .200
  • Bench-mark: geodetic; tertiary;
    BM 63.3 BM 63.3

The points to be noted are

  • Geodetic Bench Mark in giving information on height correct to + 10 Cms
  • The information in italics is providing approximate information.
  • Triangulated heights are reliable heights of the points although not correct to + 10 cms.

Further, as per map –accuracy standards, 90% of heights of sharp points on the map should be below half the contour interval. Suppose the contours are at 5 metres on 1:25000 map the heights of sharp points on this map should have accuracy of 2.5 metres.

Working Space Versus Scale of Map
Most of the times the users specify the scale of the map based on the space required on the paper so as to incorporate design features. Seldom the scale is based on the accuracy required for the measurements to be done on the map. Table 1 tells us that accuracy of 1:4000 scale is 80 cms (or 1.2 metres in linear measurement) where as the accuracy on 1:1000 scale is 20 cms. Surely, when telephone department gives a tender for the map on 1:1000 scale it does not have 20 cm in mind. This is the scale of working space which it desires. On the other hand a water – supply engineer may really need 20 cm.

An elegant solution in such a case of more paper space is naturally an enlargement of 1: 4000 map on 1: 1000 scale (four times enlargement). The enlargement will have the accuracy of its ‘parent’ scale. In our example, this parent is 1:4000 scale. The accuracy of 80 cms is good enough for all practical design jobs. A map on 1:4000 scale is certainly less formidable and is within the production reach of many organisations.

Lesson : If a smaller scale (1:4000) is adequate from the point of view of the accuracy, do not demand a larger scale (1:1000).’

This lesson saves time as well as cost.

Care has, however, to be taken that ‘original’ map/product used for enlarging has the requisite accuracy.

The proper procedure for determining the scale should be to find out the requirements of the accuracy needed by the projects and what sort of measurements for length etc are required by the user.

Example
NRSA supplies the product called Geo-coded pan scene on 1:12500 scale with the resolution of 5.8 metres.  The imagery which is supplied in photographic or digital format may have on internal accuracy of more than 8-10 meters. Any enlargement of this product (done optically or through digitisation will therefore not have better accuracy than 8-10 meters. Similary, IKONOS also will not be able to provide better plan accuracy than 3-4 metres even when the resolution is 1 metre.

Accuracy in Mapping – standards followed in India should be published
The map standards of most of the countries have evolved around their own map –making agencies. These standards are very useful in situations when the mapping jobs are outsourced or there are many agencies which are engaged in a big project. NRSA has provided a good precedence while getting the projects on Wasteland Projects and IMSD. It will be a good exercise if these standards and specifications are published for general use. [email protected] can take the initiative in this regard.

Reference to Internet sites reveals that many standards of accuracy have been worked out by various agencies in diverse areas like Digital Elevation Model, Digital Line Graph, Digital Orthophoto, Vegetation Mapping, Spot Elevations and Geospatial Positioning  Standard for Control, Drawings and Geodetic Control.
(Ref. fgdc.gov; state.nj.us;tsc.wes.army.mil; e.topo.com)

It will  be worthwhile to critically review these standards for adaptation to our local use in a given GIS project.

Geo referencing of Topographic Layer in GIS
Many GIS projects have been inflicted with inaccuracies because of indifferent (read bad) geo-referencing (GR). Some of these sources of GR are mentioned in brief. The solution for geo-referencing has to be seen under various accuracies possible with different technologies.

Survey of India – Topographical Maps on 1:25000

These maps still remain the first source of coordinates howsoever approximate those may be. On 1:25000 scale we may get the result within 6-7 metres. These can be  road junctions or village – trijunctions. Some of these details, shown on a printed map do not change over time and can be picked up even from 10-12 year old maps. In case larger scale map (larger than 1:25000) are available for the area (e.g. Town Guide  maps) the geo- referencing should be based on those to improve the coordinates.

Global Positioning System
Single GPS will not serve the purpose of referencing as there is likelihood of major error to the tune of 8 to 10 metres. Besides, the coordinates obtained by GPS have to be converted to Everest system from WGS 84 system. The single GPS receiver  is, therefore, not a reliable source for geo-reference.’

Differential GPS (DGPS) will, on the other hand, provide the requisite coordinates of the sharp features. DGPS provide the conversion from WGS-84 to Everest system is done. It is a prudent policy to have a mesh of DGPS points in the area of interest. These points will not only be useful for geo-reference but will also be very useful for subsequent surveys.

Use of Control Instrument – Total Station etc

Requisite number of geo-reference points can be obtained by bringing the control from a known Survey of India topographical station to the area of interest by standard survey methods of triangulation and traversing. Independent control can also be provided in the area by considering one of the points in the area as having (o.o) coordinates.

Other Techniques of Higher Accuracy Photogrammetry

Photogrammetric techniques are able to provide accurate ‘topography layer’. It involves use of aerial photographs and photogrammetric machines. One is able to obtain plan accuracy of 20 cms and height accuracy of 40 cms from an aerial photograph on the scale of 1:8000. NRSA as well as some private agencies also employ this technique for producing GIS data base for a large number of urban – areas/towns in India. The major disadvantage is that aerial photography is a classified document in India. It is still a better step to procure old aerial photography and commission NRSA, Survey of India or any other Government department for producing a photogrammetrically base – layer on the desired scale.

Conclusion
This paper tries to explain some very basic concepts of spatial accuracy (x, y and z) which may be required to produce a reliable GIS project. The stress has been on generating the topographical layer with proper geo-referencing keeping in mind the users of the GIS projects. The operation at the user-end will determine the specifications of the accuracy.

The objective of this paper will be fulfilled if designers of GIS projects keep to the simple requirements of accuracy so as to avoid their projects falling into the floating GIS category!.