# Understanding DEM units

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### #1 Jake See Posted 07 May 2010 - 05:05 AM

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Hi,

Greetings to all.

I have read from many sources regarding DEM files. But I still have series of questions unanswered. Hope you can help.

and I refer to the 1:250,000 DEM.

1. I don't understand why they are not consistent in the naming.
Can I know what is the difference between 7.5 minute DEM and 1:250,000 DEM? What is there no consistency in the naming?

2. Nevertheless, I think those 1:250,000 DEM are actually 7.5 minute DEM. Correct me if I am wrong. In fact the Alaska DEM are not 1:250,000 scale, so I think they should really call it 7.5 minute DEM? Am I right?

3. I read that 7.5 minute DEMs are spaced 10 or 30 meters. So is it 10 or 30 meters? How can I know (without and with parsing the DEM)? Furthermore, accordng to wikipedia, I calculated 7.5 minute = 13,890 metres (7.5 nautical miles) which makes the DEM file (1201 points) to be spaced 30.866666 meters so it is not 10 nor 30 meters? Is this correct? Or was the DEM created at exactly 30 meters?

4. Next, regarding the scale of 1:250,000, it refers to the sampled height values (in B record) only, such that e.g. 1500 is actually 1500 * 250,000 meters in real height; while the spacing remains as 30 metres (or 30.86666 metres). Am I right?.

5. What unit is the northing and eastings values of the 4 corners of the quadrangle recorded in? I get values like 369000 and 118800 which I can't seem the correct conversion to get to latitude and longitude.

Thanks for any help! thanks!

### #2 Hans van der Maarel Posted 07 May 2010 - 07:15 AM

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Just a quick reminder:

a 7.5 minute DEM measures 7.5 geographical minutes on each side. And yes, a minute does equal a nautical mile but... 1 nautical mile = 1 minute of longitude at the equator.
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### #3 Jake See Posted 07 May 2010 - 10:09 AM

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Thanks Hans, I got it.

After further searching, I have to first correct myself.

The 1:250,000 DEM files are 1 degree DEM. and the 1:24,000 DEM are the 7.5minute DEM aka 250k DEM and 24k DEM respectively. The differences can be found http://webhelp.esri......ersion) works.

### #4 Charles Syrett Posted 07 May 2010 - 11:58 AM

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Interesting topic. Things can get really confusing because measurements in longitudinal degrees become shorter relative to real ground units (metric or imperial), the further north you go -- yet DEMs "think" in square units. Over the years I've kept a running tab of the different DEMs I've used, along with their resolutions and the approximate map scales at which they can be useful for shaded relief. I've attached this as an Excel file. It's full of gaps and some of the values on it can be disputed, but some folks may find that it comes in handy, as I have.

Charles Syrett
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### #5 David Medeiros Posted 07 May 2010 - 12:35 PM

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Interesting topic. Things can get really confusing because measurements in longitudinal degrees become shorter relative to real ground units (metric or imperial), the further north you go -- yet DEMs "think" in square units. Over the years I've kept a running tab of the different DEMs I've used, along with their resolutions and the approximate map scales at which they can be useful for shaded relief. I've attached this as an Excel file. It's full of gaps and some of the values on it can be disputed, but some folks may find that it comes in handy, as I have.

Charles Syrett
Map Graphics
http://www.mapgraphics.com

I work with a lot of those same data sources and have similar notes scribbled in various locations, nice to have them all together along with notes on max map scales. Thanks!
GIS Reference and Instruction Specialist, Stanford Geospatial Center.

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### #6 Charles Syrett Posted 07 May 2010 - 12:58 PM

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I work with a lot of those same data sources and have similar notes scribbled in various locations, nice to have them all together along with notes on max map scales. Thanks!

True Confessions: Mine, too, was scribbled on paper until this morning, when I finally got around to making an Excel version because I saw this thread! Anything that you (or anyone else) can add to this, or improve on it in any way, would be most appreciated.

Charles Syrett
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### #7 Jake See Posted 07 May 2010 - 11:22 PM

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Thanks Charles for sharing =)
Unfortunately, I can't seem to open the zipped xls file [edit: managed to open by first ungzip, then unzip],

I noticed that many DEMs come sets of in "c, n, s, e, w" versions I don't understand how to use them correctly.

It comes with one east and one west DEM.

As per DEM specifications, each DEM represents a square area; hence, stiching the 2 DEMs east and west together gives a rectangluar area. But on geobase.ca, this 074G07 is clearly a squarish quad. Why is this so? Is it because UTM mapping of the quadrangle makes rectangular physical areas appear square on the UTM map?

(Sorry, I have never in my life so far studied Geography or GIS/Mapping, please bear with me.)

### #8 Charles Syrett Posted 08 May 2010 - 11:01 AM

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Jake, you just happened to select an example which perfectly illustrates the point I made (above) relative to E-W measurements in degrees becoming shorter as you move northward. The particular sheet you're looking at ("quadrangle" is an American term, BTW) is far enough north that the shape of the sheet is almost "squarish", because 30 minutes of longitude (the E-W extent of a Canadian 1:50K sheet) has so decreased that it's almost the same as 15 minutes latitude (the N-S extent of a Canadian 1:50K sheet). But for the DEM, which is unprojected, the two halves (east and west) are each square (15 minutes by 15 minutes), adding up to an overall sheet which is twice as wide as it is high -- exactly as it would be for a DEM of a southern latitude.

Hope this helps!

Charles Syrett
Map Graphics
http://www.mapgraphics.com

Thanks Charles for sharing =)
Unfortunately, I can't seem to open the zipped xls file [edit: managed to open by first ungzip, then unzip],

I noticed that many DEMs come sets of in "c, n, s, e, w" versions I don't understand how to use them correctly.

It comes with one east and one west DEM.

As per DEM specifications, each DEM represents a square area; hence, stiching the 2 DEMs east and west together gives a rectangluar area. But on geobase.ca, this 074G07 is clearly a squarish quad. Why is this so? Is it because UTM mapping of the quadrangle makes rectangular physical areas appear square on the UTM map?

(Sorry, I have never in my life so far studied Geography or GIS/Mapping, please bear with me.)

### #9 Jake See Posted 09 May 2010 - 12:04 AM

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Jake, you just happened to select an example which perfectly illustrates the point I made (above) relative to E-W measurements in degrees becoming shorter as you move northward.

Sigh, that was really confusing for beginners like myself. Luckily, we have all you awesome folks =)
Many thanks!

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