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View Full Version : DEM Maps - Proper Scale

devin
10-25-2007, 11:16 AM
Hi,

I've been reading through some prior threads in order to properly import a digital elevation map into LW. I think I've got that part correct now (I hope). I need to ensure that it's the proper scale, however, and thought I'd post. I pulled a 7.5 minute tile/area. How do I actually determine the proper distance that this covers. I've seen a number of figures such as 1cm=240m, 1 min.=1.15 miles, etc. The problem is that these different methods give me different results. Is there a clear cut method for this that's accurate? Should the model pulled from the DEM data already be accurate? If the elevation is actually correct, the distance along the remaining axes should be true as well, right?

To add to the confusion, one tutorial had me pull the map at NED 1/3" and then convert it using 3DEM to a UTM Projection. This causes the originally square tile to become more rectangular (taller than wide). Is the converted projection map the true proportion? This is all a little Greek to me.

Thanks in advance for any insight.
Devin (the confused)

devin
10-25-2007, 11:35 AM
I somehow overlooked it multiple times before but I did find a measuring tool on the USGS site. It's in feet on a large scale map so I don't see it being extremely accurate but it's better than what I had before. It looks as if the 7.5 min. tile should be about 7.183 miles W x 8.651 miles H. Does this seem about right? I'd still be curious about any insight into the post above.

Thx.
Devin

MicroMouse
10-25-2007, 05:08 PM
http://mcmcweb.er.usgs.gov/drc/dlgv32pro/index.html

http://www.usna.edu/Users/oceano/pguth/website/microdem.htm

Both of these programs have measuring tools.

I only partly understand what happens with digital mapping of the earth.

The earth is not a sphere but an ellipsoid (oblate spheroid) flattened at the poles and bulging at the equator. Each of the various map types have an associated datum which is used to correct the measurements along the ground depending on where on the planet the map is from.

At the equator, 3 arc seconds is about 280 feet.

For a 7.5 minute DEM:
7.5 min * 60 sec/min * 280 ft/3 sec * 1 mile / 5280 feet = 7.95 miles without correcting with a datum. So your measurements are in the proper range.

7.5 minute DEM files are usually sampled at either 30 meters (about one arc second) or 10 meters (about one third of an arc second).

NED 1/3" files have a sample distance of one third of an arc second or 10 meters.

Wayne

archijam
10-26-2007, 02:53 PM
Wayne Rocks!

j.