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Greetings to all! First post in here.

Firstly, I am a complete noob in geography. But I have to do some programming work with DEM files.

I got some DEM files from http://geobase.ca/geobase/en/find.do?produit=cded the 1:50,000 version and read in the data. According to the DEM specifications, there is a quote:

"... a 4x2 array containing the ground coordinates of the quadrangle boundary for the DEM. The coordinates of the quadrangle corners are ordered cloclwise direction beginning with the SW corner. The array is stored as pairs of eastings and northings."

Question: 1. Is easting and northing same as UTM metric? 2. Are UTM coordinates supposed to be written as easting-northings or northing-eastings? 3. What is the standard unit for UTM coord? meters? arc seconds? In the DEM file i have, I figured it is in arc seconds. 4. How to I convert, specifically for this DEM (see above) I have, from the "4x2" array to meters?

Below is one of the DEM header data i extracted. Which this data, specifically, the SE coord (2nd line) and the 4x2 array (SW, NW, NE, SE coord) is it sufficient to point on the map where this quadrangle is? I tried very hard to map this data to google earth but i cannot seem to work out the maths.... hope someone can enlighten me. Thanks in advance!

DEM Name: 092g01DEMe
SE Coord: -12215 0.0000, 49 0 0.0000
DEM Level Code: 1
Ground Reference System: Geographic
Ground Reference Zone: 0
Ground Unit: arc_seconds
Elevation Unit: meters
Ground Resolution (lat, lng, elev): 0.75, 0.75, 1
Elavation Array Szie: 1201 x 1201
Percentage void: 0
SW Coord: -440100, 176400
NW Coord: -440100, 177300
NE Coord: -439200, 177300
SE Coord: -439200, 176400
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1 Answer 1

up vote 3 down vote accepted

Is easting and northing same as UTM metric?

Depends on the situation. In this case, it seems like easting and northing are absolute arc-seconds rather than relative meters from an arbitrary origin.

Are UTM coordinates supposed to be written as easting-northings or northing-eastings?

UTM coordinates are usually written as (easting, northing) to mirror the cartesian (x, y) convention.

What is the standard unit for UTM coord? meters? arc seconds? In the DEM file i have, I figured it is in arc seconds.

As far as I know, the standard unit for UTM coordinates is the meter.

How to I convert, specifically for this DEM (see above) I have, from the "4x2" array to meters?

Since the ground unit is arc-seconds:

SW Coord: -440100, 176400 
NW Coord: -440100, 177300 
NE Coord: -439200, 177300 
SE Coord: -439200, 176400 

Try dividing the coordinates by 3600 to get the values in degrees:

SW Coord: -122.25, 49.00 
NW Coord: -122.25, 49.25
NE Coord: -122.00, 49.25 
SE Coord: -122.00, 49.00 

If we interpret this as 49N, 122W with 0.25-degree offsets we get a location on/near the border between WA and BC. Converting this to meters for use in Google Earth is a little trickier. I don't know what Google Earth expects (UTM or an arbitrary local tangent plane), but you can try using http://nearby.org.uk to convert to UTM (Zone:10U E/N:573142,5427937 for 49N, 122W).

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Wow! Many thanks. All makes sense now. Your calculated location is also spot on! Except, i lost you at "UTM or an arbitrary local tangent plane". I have tried to use many online conversion tools but without success. I think it is mostly my lack of knowledge, but some sites have various disclaimers saying their conversion is based off some outdated specifications etc. Thanks again! –  Jake Oct 13 '10 at 22:38
    
oh btw, if you see this again, why would the specs say the 4x2 array is "pair of eastings and northings" instead of pair of lat lng"? Which in this case it is definitely so -- absolute arc-seconds. Isn't it confusing to use the terms interchangeably? –  Jake Oct 13 '10 at 22:41
    
A local tangent plane is just a flat-Earth projection with an arbitrary origin. The coordinate conversion tools at nearby.org.uk are very useful. Are you trying to convert the DEM to KML? According to the KML spec It seems like the coordinates need to be in decimal degrees. –  Jaime Soto Oct 14 '10 at 2:26
    
Thanks for the info. I'm not converting to KML. I am using the DEM for engineering control application. I want to traslate the coords to meters so I can compute stuff like velocity and acceleration for a UAV. Not sure if a DEM is a very good choice though... –  Jake Oct 15 '10 at 11:58
    
You can try to re-project the DEM using a GIS viewer/editor (ArcGIS, MapWindow, QGIS) or through the command line with gdalwarp: gdalwarp -t_srs '+proj=utm +zone=10 +datum=WGS84' <source file> output.tif –  Jaime Soto Oct 15 '10 at 13:39

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