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GIS Frequently Asked Questions (FAQs)

What projection should my data be in?

The USDA Service Center Agencies have decided to use UTM NAD83 as the default projection for all geospatial data. There are 3 UTM zones in Montana. Each county has been assigned a default UTM zone. Data in a different projection should be named accordingly, i.e. soils_a_mt000_z12.shp

What is the difference between NAD27 and NAD83?

About 200-500 feet, depending on where you are. There are 3 characteristics that define geospatial data. The spheroid is a rotated ellipse which models the shape of the earth. The datum takes the spheroid and describes reference points in relation to the center of the earth. The projection uses the datum and “projects” the spherical coordinates to planar (flat) coordinates. The North American Datum of 1927 (NAD27) uses the Clarke 1866 spheroid. As you might imagine, technology has improved greatly since 1866, so the North American Datum of 1983 (NAD83) uses the GRS 1980 spheroid, which was calculated using satellite data. In a nutshell, the NAD83 data is much more accurate then the NAD27 data. Some field offices may still have some NAD27 data on their servers. This data should be removed to avoid confusion.

How come these layers don’t line up?

In ArcView 3.3, the shapefiles had to be in the same projection to overlay properly. ArcGIS will project on the fly for display purposes, so layers with different projections should still line up. This feature is dependent on the shapefile having a .prj file which defines the projection. Shapefiles created in ArcView do not have a .prj file. If the projection is known, a simple way to create a .prj file for these old ArcView files is to copy the .prj file from a shapefile with the same projection. To do GIS analyses in ArcMap, layers still need to be in the same projection, even though they line up properly for display.

What are all these extra files with the shapefile?

A “shapefile” is actually a collection of several files. At a minimum, there must be a .shp, which holds the geometry, a .dbf, which holds the tabular data, and a .shx, which ties it all together. There may also be several other files such as .prj, .sbn, .sbx, .avl, .doc, and .xml. When moving a shapefile, be sure to move all the components. When emailing, zip the files to keep them together.

Why do the files in f:\geodata have such strange names?

File names have been standardized for several reasons.

  1. The naming convention creates nationally unique filenames.
  2. Employees can travel anywhere and use local data because the data structure is familiar.
  3. Updated data can be pushed down automatically to overwrite existing files without disrupting existing projects.

Can I copy geodata to my hard drive so it loads faster?

Yes. Not only will the data load faster, but if you have a laptop, it will allow you to work remotely. The majority of the data in f:\geodata is static, but when it is updated, an announcement will be made.