The MCHP GIS MANUAL - Datums

         

Introduction   Contents


 USING ArcGIS: 
   
1. Basic Use of ArcMap 
   Creating a Map 
   Data Frame Tools
   Data Frame Properties
   Tables and Charts
   Layout View
   Saving Map Documents
   Activity 1

2. Map Projections
   Projections
   UTM
   Datums
  
3. ArcCatalog 
   Basic Uses    
   
4. ArcToolBox 
   Basic Uses    
   
5. Adding Spatial Information
   Georeferencing
   Adding Non-Spatial Data
   Dbase Files
   Activity 2

6. Selecting Data
   By Graphics
   By Location
   Activity 3

7.  Exporting Maps
   Exporting
   Activity 4
  
 
FURTHER INFORMATION:
   General Information
   Data Sets

DATUMS

Here is what the ArcView help files say about datums:

A datum is a set of parameters defining a coordinate system, and a set of control points whose geometric relationships are known, either through measurement or calculation. One part of defining the coordinate system is the spheroid used to approximate the shape of the earth.

A spheroid is defined by a radius and an eccentricity. These two constants are used as inputs to the equations which calculate a projected coordinate from a coordinate in decimal degrees. When a projection is created, it is associated with a default spheroid so that these constants will be available. This default spheroid varies from projection to projection, but is usually the SPHERE for small-scale projections and CLARKE 1866 for large-scale projections.

Certain predefined projections (for example, those stored in default.prj in ArcView's etc directory which are displayed as "standard" projections in Projection Properties), are associated with specific spheroids based on their use. For instance, the "State Plane - 1927" projections are associated with the CLARKE 1866 spheroid (with the exception of Michigan), the "State Plane - 1983" projections are associated with the GRS 80 spheroid, the "New Zealand National Grid" projection is associated with the INTERNATIONAL 1909 (also known as the INTERNATIONAL 1924) spheroid, the "Great Britain National Grid" projection is associated with the AIRY spheroid, and the National Grids of Malaysia, Singapore, and Brunei are associated with the EVEREST spheroid.

When a projection associated with a certain spheroid is used, ArcView assumes that the decimal degrees data being projected was collected in a datum based on that spheroid. So while ArcView has no knowledge of datums per se, it does know about spheroids, which are part of the definition of a datum. Therefore you do need to know what datum your data is in, and set the spheroid of the projection accordingly. This can be done either through the Projection Properties dialog or by using Avenue.

 

Projections  

Contact: Charles Burchill       Telephone: (204) 789-3429

Manitoba Centre for Health Policy
Department of Community Health Sciences, University of Manitoba
4th floor Brodie Centre
408 - 727 McDermot Avenue
Winnipeg, Manitoba R3E 3P5       Fax: (204) 789-3910
Last modified on Friday, 25-Aug-2006 08:06:00 CDT