Coordinate Systems and Datums
Changing projection in Idrisi
GIS in EA 2003
Objective: Carry out datum and projection changes in Idrisi.
A GIS user often has to figure out the datum and projection of spatial data, and transform these data to the most useful coordinate system. The folder Wk-2 projections contains the needed map data. Copy the
Wk-2 projections folder to your C:\temp folder.
Look up the metadata on the vector images Thurston basins (a drainage basin map), and Thurston geology (a surficial geology map). Notice both have Plane as the Reference system . These maps actually have one of the following common reference systems.
Latitude, Longitude in NAD27 or NAD83
State Plane coordinate system in NAD27 or NAD83 in WA north or south zone.
Universal Transverse Mercator in NAD27 or NAD83.
To view these 2 maps open the Basins vector map with the qualitative 256 palette and no legend. Add the
Geology vector map with the qualitative 256 palette. Zoom out using the zoom out button on the composer. Eventually both maps will be visible. The difference is due to difference in datums. One is
NAD27 the other NAD83, and both are in feet.
Given what you have learned about datums and coordinate systems determine which applies to the
Thurston County maps. (Use your experience from ArcView the previous quarter.)
Thurston basins is: _______________
Thurston geology is: ________________
Open the metadata for the basins map and change the Reference system to the one you determined. Use the scroll arrow which will appear when you click on the Reference system line and select the appropriate abbreviation for the reference system. (Note: this does not change the reference system, it just changes the name give in a line of text in the metadata.) Next change Reference units to the appropriate units used on the reference system (meters for UTM and state plane NAD83, feet for state plane NAD27) then Save the metadata with the save command in the file menu. Important note, Idrisi will not accept feet for computations in state plane NAD83. To change a map in feet and NAD83 enter meters as the
Reference units and change the Unit distance in metadata from 1 to 0.304800 (the number of meters in a foot).
Do the same for the geology map. Change the metadata to the appropriate system and units and save the metadata. Important note, Idrisi will not accept feet for computations in state plane NAD83. To change a map in feet and NAD83 enter meters as the Reference units and change the Unit distance in metadata from 1 to 0.304800 (the number of meters in a foot).
In the Idrisi Reformat menu select Project (for Projection) and fill in the dialogue box so the new map will be projected into United States transverse Mercator NAD83 (us83tm10). Do this for both maps.
Use add layer in the composer to bring both maps up on the screen and see if they overlay more or less correctly. Use the zoom tool to look at a shoreline area. What typical data entry problem to you see in terms of the shore boundary?
You can use this procedure to change coordinate systems on any raster or vector map in Idrisi.