Horizontal Datum Issues & MICRODEM

All open maps will have the same primary datum, set on the maps tab of the options form. The datums should be selected before opening data, and you probably cannot set the primary datum with open data sets. All data displays, and coordinate entry, use that datum.

You can display coordinates in two datums as you roam with the mouse. This is also set on the maps tab of the options form. This can include your primary datum and and datum on which the data was digitized, or any choice of two datums. The datums should be selected before opening data.

All DEMs and imagery (including scanned maps) will have a datum transformation applied when you open them. Whatever the datum used to digitize the data sets, they will be displayed on your primary datum. The datum shift moves the SW corner point (for a DEM) and the NW corner point (for an image) to the new datum, and then assumes the horizontal and vertical spacings remain the same in the new datum, and there is not a significant rotation between the two datums. If you do insist on using a primary datum other than WGS84, the raster imagery (DEMs and imagery) should work correctly. When a DEM or image does not explicitly state its datum on import, you will be prompted to enter it.

Vector data (e.g. Shapefiles, and TIGER files) will generally not have a datum shift applied. It is up to you to select the correct primary map display datum for the vector data coming in. Mixing and matching datums with vector data will lead to misregistration.

You can do a datum shift for Shapefiles on the data manipulation form, Resample, Shape file menu choice.  You must transform to WGS84 Lat/Long coordinates before using them.

Saving data and datums:

You can get the digitizing datum by looking at the metadata for your files, or the Analyze header menu choice.

Only in exceptional cases should you use another primary datum than NAD83/WGS84, and then you must be careful.  The problems will come with plotting vector and GIS database files, which in general will not do a datum shift. In addition, some options in this program assume WGS84 as the datum.

See also:


Last revision 8/16/2015