Table 1: Proposed Datums 
  
  
  
  
  
  
  
  
  
  
  
  
GIS Region Datum/Horizontal Remarks 
Global Entire Earth A OS 
Regional Africa African Datum Rename and Adjust 
» Asia Asian Datum Rename and Adjust 
" Australia Australian Datum "d -——-— 
x Europe European Datum Include the former 
USSR and Readjust 
a North America NADS83 +++ => io) Pre 
= South America South American Datum Readjust 
Local GIS Entire Earth Same as for Regional | -———— 
  
  
  
  
  
Datum/Reference Surface 
The first thing to be considered in GIS planning is to choose 
a datum for referencing information 
into a location (longitude, latitude, X, Y) and elevation 
(from mean sea level, i.e., orthometric height or from the 
reference ellipsoid, i.e., ellipsoidal height). There are about 
fourteen main reference ellipsoids used around the world for 
surveying and mapping purposes. Selection of the 
appropriate reference ellipsoid for location and vertical 
datum for elevation affects the accuracy of GIS products. 
Many geodesists suggest a globally best fitting geocentric 
ellipsoid for the earth. Because of heterogeneous 
distributions of geodetic controls in different continents, one 
datum would not fit better regionally for all the continents. 
Therefore, seven different geocentric datums (six for six 
continents, and one for global coverage) have been suggested 
for global, regional, and local GIS (Table 1). 
The vertical datum should be one geoid for the entire Earth, 
and all height computations should be based on that one 
vertical datum. 
Selection of Mapping Plane (Map Projection) 
The second step in planning a GIS is selection of a suitable 
mapping plane. Approximately 27 different types of map 
projections have been used around the world for surveying 
and mapping purposes. An appropriate combination of 
reference curved and plane surfaces can produce an optimal 
GIS. An unmatched mapping surface could cause severe 
errors in coordinates [Acharya, 1990]. 
Map Scales and Map Numbering 
An optimal surveying and mapping system is an integrated 
GIS. The task of making an integrated GIS is almost 
impossible and cost ineffective if different agencies within a 
region or zone use different reference frames, and base 
maps. To obtain an integrated GIS, a well defined reference 
frame and map numbering system must be adopted within 
countries or states or zones by all agencies where GIS work 
is performed. For detailed explanation please refer to 
[Acharya, 1990]. 
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Geodetic Control 
A minimum of three control points are needed to uniquely 
define the mapping plane surface, i.e., to tie the map frame 
to the map projection plane and ultimately to the reference 
surface (datum). If mapping is to be done at a scale of 
1:500, then control stations are needed for about every 200 
meters [Acharya, 1990]. This means a very large number of 
control stations are needed for a large scale GIS. These 
stations are needed to present map features in correct 
relationship to each other, to the reference surface, and, 
ultimately, to the earth’s surface. Two kinds of control 
stations are needed, horizontal and vertical. These controls 
maintain correct scale, position and orientation of the map. 
The horizontal as well as vertical control points become the 
framework on which map details are compiled. An accurate 
and optimal framework also permits map details to edge 
match perfectly from one sheet to the next and also to 
prepare mosaics or seamless maps. An example of 
minimum numbers of geodetic controls required for regional 
GIS is computed and given in Table 2. 
From Table 2, one can see how huge numbers of geodetic 
control points are still needed to cover the continental area 
of the Earth. Without the use of GPS technology, another 
half century may be needed to accomplish this task. The 
number of geodetic controls required for the local type of 
GIS increases exponentially but local GIS may not cover the 
entire country or region. Usually local GIS are prepared for 
urban areas and areas of special interest. The number 
geodetic controls required depends upon the scale of base 
maps to be prepared. 
INFORMATION COMPONENT 
This is a major component of a GIS system. Information is 
collected using three techniques: 
® ground survey method 
® photogrammetric method (airborne) 
® remote sensing (air and space borne) 
Digital cartographic data is generated by digitizing 
conventional hard-copy maps, photogrammetric 
stereocompilation, softcopy photogrammetry (digital 
photogrammetry), satellite remote sensing, and different 
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