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I am a GIS hobbyist trying to create a merged DEM of the St. Louis region in ArcGIS Pro 2.0. So far I have been successful in creating two mosaics, one of the Illinois side of the Mississippi River and one of the Missouri side. I created both by combining county DEM rasters from two different web sources. Here is an image of the two combined rasters:

Unmerged DEM

Please note that originally these were two separate images that I combined after exporting from ArcGIS. This is not representative of where they are spatially. I would have them as two separate images if not for StackExchange's prohibition of more than two links and images in a question for new users.

The Issue

The problem is what happens when I put the Illinois and Missouri DEMs together in a mosaic: Troublesome merge

As you can see, the Illinois side has higher elevation values than the Missouri side. When I compare the values of overlapping areas, the Illinois side is about 300 higher. The trouble is, the value increase in overlapping areas is not consistent. Sometimes it's about 250, other times 375.

I imagine that what I need to do is lower the elevation of the Illinois DEM or raise the elevation of Missouri, but I am not sure how. I have not been able to find anyone who has asked about an issue like this before. I would still like the DEM to be accurate after adjusting the elevation, but absolute accuracy is not essential. My goal is to create an elevation map of the St. Louis region for illustrative purposes.

Data Sources

Illinois

Specifically, I downloaded the derivatives for Jersey, Madison, St. Clair, Monroe, and Randolph Counties. The files were originally in .ovr format.

Missouri:

Missouri had two data sources, both from the same website. First I downloaded DEMS for each county from here. Those were geodatabases. I then had to download individual tile DEMS from here to cover gaps between the borders of each county. Those files differed in format; some were adf and some were geotifs.

Apologies for the awful link formatting. I am not allowed to post more than two links yet.

UPDATE

It turns out there was a unit issue, as explained by the top answer below. I just want to share a guide here for anyone looking at this in the future who has a similar issue with units.

  • Also, if it is useful, here are links to the web sources for the DEM data: Illinois. Specifically, I downloaded the derivatives for Jersey, Madison, St. Clair, Monroe, and Randolph Counties. The files were originally in .ovr format. – Bosterm Jul 10 '17 at 1:15
  • Missouri had two data sources, both from the same website. First I downloaded DEMS for each county from here. Those were geodatabases. I then had to download individual tile DEMS from here to cover gaps between the borders of each county. Those files differed in format; some were adf and some were geotifs. – Bosterm Jul 10 '17 at 1:17
  • Welcome to GIS SE. Thank you for taking our Tour. Please do not comment on your own post. All relevant information should be in the body of the question, including the exact software you are using. – Vince Jul 10 '17 at 1:36
  • I had to post them as comments because StackExchange would not let me put any further links in my original post. – Bosterm Jul 10 '17 at 1:54
  • You can put the URL as text. All comments aren't always shown, and folks here aren't looking for content in comments, so it hurts the quality of your question. – Vince Jul 10 '17 at 2:11
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The Missouri data is in metric (metres) whereas the Illinois data is in feet. That is why things aren't lining up. I know this because I was working with this same data last week. Convert the Illinois data to metres by dividing the raster by 3.048. Or conversely convert the Missouri data to feet if desired.

  • Hey thanks, I'm currently running the conversion on the Illinois DEM to see if this works, which will take some time because it's a large DEM. I will let you know if it works. The two sources using different units would certainly explain why the difference varied. – Bosterm Jul 10 '17 at 18:26
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    Alright great that seems to have worked! It's still taking some time to combine everything, but that's just because everything is so big. I can certainly handle that. To anyone reading this who is dealing with a unit issue like this in the future, follow this guide. – Bosterm Jul 12 '17 at 3:55

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