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I have a hard time understanding what ASCII files are. I have this DEM in asc. I try to import them into ArcMAP 10.3 and they appear in my naked eyes as images however what I found on internet is ascii .txt.....what are they? Is there other extensions called ascii too? I want to use this DEM for HEC-GeoRAS and I understand that HEC-GeoRAS need DEM in TIN as data input. I have no idea what to do with this ascii file.

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ASCII files are text files that can be readable in any text editor and you can check their contents. However, the ASCII files may represent raster data, such as DEM, but they should be formatted in a specific way in order to be able to convert them into raster DEM. You can use ASCII to Raster tool in ArcToolbox, but the data should be formatted in the following format:

NCOLS xxx
NROWS xxx
XLLCORNER xxx
YLLCORNER xxx
CELLSIZE xxx
NODATA_VALUE xxx
row 1
row 2
.
.
row n

An example of valid ASCII format is like this:

NCOLS 480
NROWS 450
XLLCORNER 378922
YLLCORNER 4072345
CELLSIZE 30
NODATA_VALUE -32768
43 2 45 7 3 56 2 5 23 65 34 6 32 54 57 34
35 45 65 34 2 6 78 4 2 6 89 3 2 7 45 23 5 ...

If the ASCII file that you have has a different format from the above, then it does not represent an ASCII that can be converted to raster. You need to download a valid ASCII file that represent a DEM, or raster DEM of Geotiff format for example to avoid any conversion process. But since we don't know what kind of DEM you have, it is difficult to help you more until you provide additional information.

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