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mosaic raster (DEM)

October 19 2010 | 3 comments
Categories: Map Data

Hi there,
I have two DEMs I want to mosaic. They are both situated on the border between Canada and the US. Before I can mosaic I need to project them both into the same coordinate system. I would like to use NAD83 UTM Zone 10N. The US DEM is in State Plane and probably uses ft for z values (which is probably causing the problem). What is the correct way to project the data into UTM and then have it mosaic correctly. My goal is to create smooth contours that cross the border using the combined datasets.
thanks for help.

Mapping Center Answer:

You need to do two things: 1) convert one of the rasters to the same Z units as the other, and 2) project one of the rasters to the same coordinate system as the other. Once they are both in the same coordinate system and have the same units for the Z values, you can mosaic them. 

For the first step, you can use the Times tool in the Math toolset in the Spatial Analyst toolbox.  To convert from feet to meters, multiply the raster in feet by 0.3048. To covert from meters to feet, multiply the raster in meters by 3.28084.

For the second step, you can use the Project Raster tool in the Projections and Transformation - Raster toolset in the Data Management toolbox.

But looking at your data you have a bit more of a challenge than that.  You also have different cell resolutions (18m for the Canadian data and about 32m for the US data).  And even if you do the conversion of feet to meters, the pixel values along the boundary between the two data sets are not continuous - most likely because the data are at differing resolutions.  So I think the better thing to do is to find a different, contiguous data set.

For example, you could download the ASTER data (from which is continuous data at 30m resolution for your area in both the U.S. and Canada.

thx posted by Rob Ostwald on Oct 29 2010 12:21PM
I will take a look at that data. Thanks for your response.
Worked for me! posted by Aileen Buckley on Oct 29 2010 1:42PM
I tried the data and they worked for me! Hope they work for you, too.
By the way... posted by Aileen Buckley on Oct 29 2010 1:44PM
Can you please send me an email when you get a minute -- our emails back to you are bouncing. Thanks!

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