Ask Norm - January 1999

Ask Norm - January 1999

Q. I've been looking for a list of NAD 83 zone codes by state and county...right now we're using a poor quality graphic map that is very hard to discern border counties. Can you help? -Dave B.

Update: The USGS does not have the paper map. They will have a digital map in the fall of 1999. In the meantime, you can see our scanned State Plane Map or use our State Plane Zone Locator.

A. Here at Mentor Software we have a map purchased from the National Geodetic Information Center National Oceanic and Atmospheric Administration

Title: Index of State Plane Coordinate (SPC) Zone Codes (NAD 1983)

It is a map with all of the US with states, counties, and State Plane Codes.

For more information, You can check out their web site:
http://www.ngs.noaa.gov

or contact them:

National Ocean Service
NOAA 1315 East-West Highway
Silver Spring, MD 20910-3282
Phone:(301) 713-2692

Q. Does Tralaine convert DEM files?

A. Tralaine does not convert DEM files in their original form. Tralaine can usually be used to convert the DEM information once it is converted to the more useful X, Y, and Z form. The X, Y, and Z form is the form which DEM information must eventually be converted before it can be used by programs such as contour programs.

There is an important reason why Tralaine does not convert DEM files. DEM files essentially are grid based. That is, elevation values are lumped into a regular grid where the DEM file header defines the horizontal location of, for example, the southwest corner of the grid and the distance between grid elements. Therefore, the horizontal position of each elevation value is determined by its grid distance from the appropriate corner of a rather large grid. This horizontal position for each elevation value can only be accurately computed for the grid upon which the DEM file is based. This is accomplished by using the grid parameters included in the header of the data file.

One could, of course, simply convert the reference coordinate assigned to the sourhwest corner of the grid. However, due to the nature of cartographic conversions, there is no guarantee, in fact it is highly unlikely, that the new horizontal positions assigned to each horizontal value can be accurately specified within the framework of the grid set up which is ingrained in the DEM file format.

To understand this, consider that conversion of UTM coordinates to, lets say, State Plane coordinates involves scaling which is not necessarily uniform, and rotation which is not necessarily uniform. Therefore, the conversion of a DEM file from UTM to State Planes coordinates could only be accomplished in an appproximate manner. Conversion of the resulting X, Y, and Z coordinates can be accomplished very precisely and is the preferred method of accomplishing the goal.

Another related issue is the conversion of a DEM data after it has been contoured. Again, the best technique is to convert the X, Y, and Z file, in case before contouring. By converting before contouring provides you with the ability to obtain contouring intervals which are related to horizontal X and Y units. That is, after contouring is completed, converting the contour interval from, lets say, meters to feet is very difficult, perhaps impossible.

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