How to obtain, define projection, mosaic, and reproject SRTM DEM data for areas of the world (outside the US)
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Description: Here you will work through a process that will allow someone with minimal DEM experience to obtain DEM’s for any section of the world (I believe) at a 90 meter resolution. For US DEM’s, there are higher resolution rasters available.
Objectives: Student will be able to
- Acquire DEM data from the SRTM mission for anywhere in the world
- Work with ArcToolbox to convert ASCII file to Raster Format
- Work within ArcToolbox to Define a projection of a raster (Geographic, Lat/Lon), and then to project a defined projection to a local coordinate system (Such as UTM)
- Mosaic multiple DEM’s together to form a continuous raster
Keyword: DEM, World, Raster, SRTM, 90 Meter, Elevation, Topography, Watershed
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Step 1: Download the DEM data
1) Go to
2) Used the Google Earth Viewer (Download SRTM KML file) to browse to area of interest,
3) To download the DEM within the square boundary, click on green triangle in center
4) A page opens, chose FTP: Download the DATA tile in zipped ARCASCII format (from London server).
5) Save to D: drive
6) Repeat steps 3-5 for your entire study area
7) Unzip the files to same folder (Extract to here)
Step 2: Convert the ASCII File to a Raster
1) Go to Tools Extensions and check the box for Spatial Analyst
2) ArcCatalog ArcToolbox Conversion Tools Raster ASCII to Raster (some help was found here
**when you are adding your ASCII make sure that the file type at the bottom is in ASC format**
3) Output should be a binary grid
4) This conversion allows to you bring the data into ArcGIS
Step 3: Define the projection of the file
1) Define the projection of each of your Raster files. Open ArcToolbox Data Management Tools Projections and Transformations Define Projection
2) Select Geographic Coordinate System World WGS 1984
Step 4: Mosaic your DEM’s together
Mosaic is another word for join, especially when referring to rasters.
1) Go to ArcCatalog and create a personal geodatabase in your D: drive folder by right clicking New File Geodatabase
2) Back in ArcMap, use the Toolbox again, follow Data Management -> Raster -> Mosaic to New Raster..." function
(Signed 16 Bit, GCS WGS84 coordinate system) and output to the personal geodatabase that you create and give it a name
2) Check the DEM – ask yourself – do my numbers make sense? Are they feet or meters?
Step 5: Project the raster into a coordinate system
1) Figure out what local coordinate system you need your data in – likely UTM.
2) Go here and find the zone.
Use NAD 27 with the appropriate zone most likely.
3) Next we will project the raster to a local coordinate system. For example – Panamawas converted to NAD 1927 UTM Zone 17
4) Data Management Projections and Transformations Raster Project Raster
5) If you data frame is still in geographic coordinates – you may want to right click on Layers Properties Coordinate Systems Predefined Projected Coordinate Systems UTM….(pick the same as the one you chose for projecting the raster.
6) You should be all set and ready to use spatial analyst, etc.
Feel free to add to this document to make things more clear for others.
Various sources and notes are listed below.
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Notes and Comments.
Please leave a note or comment and reupload the document if you use it, make it better, or just want to share an insight from your experience working with this documentation. ////////////////////////////////////////////////////////////////////////////////////////////
Then, I used the FAQ at this link:
In that FAQ, I found that to use this data in ArcGIS I need to go to ArcCatalog ArcToolbox Conversion Tools Raster ASCII to Raster
I did the conversion and was able to bring the 90 M SRTM data as a raster into ArcGIS.
Kim do this one -----//Alternate/// If you download Arcinfo ASCII format use the "ASCII to Raster..." in Arc 9 Toolbox to convert to binary GRID.
For mosaicing the tiles I've found that the "Data Management -> Raster -> Mosaic to New Raster..." function (Signed 16 Bit, GCS WGS84 coordinate system) and output to a Geodatabase Raster works best.
AC – 10/4/07
Note: An alternate location to purchase what appears to the same data for about $40 is through the Mapmart portal at:
AC – 10/5/07
From the ESRI Knowledge Base….
Clean SRTM data are available!
Author Nicholas Tripcevich
Date Nov 14, 2004
Message SRTM update
First the Bad News:
A "finished" version of SRTM data has been released by USGS/NGA with some corrections particularly with regards to hydrology, however the no-data void problem persists. The "finished" data isn't yet available on the seamless server, so we purchased 3 CDs from the USGS assuming that the voids would be gone. The smaller voids (speckling) are resolved, large water bodies have been burned flat into the DEM, and the data conforms to DTED, but the big no-data voids are definitely still there.
The "finished" processed data and a brief description can be found here
The Good News:
The Consultative Group for International Agriculture Research (CGIAR) has further processed the "research" grade SRTM data to fill the voids and smooth the seams, and they are generously giving these worldwide data away free as GeoTIFF or ArcInfo ASCII.
Their SRTM data website is
See their website for a technical article about how they patched the voids. Essentially, their technique was a laborious process in Arc/Info workstation TOPOGRID that involved going back and forth raster to vector and back to raster using a 10 m contour interval.
CGIAR is currently the best free source of processed and consistent SRTM elevation data (Nov 2004). If you want to know where they've patched voids (and where the data are therefore not reliable) download their "Mask" raster as well.
If you download Arcinfo ASCII format use the "ASCII to Raster..." in Arc 9 Toolbox to convert to binary GRID.
For mosaicing the tiles I've found that the "Data Management -> Raster -> Mosaic to New Raster..." function (Signed 16 Bit, GCS WGS84 coordinate system) and output to a Geodatabase Raster works best.
AC – 10/22/07 AM
I worked with Matt to get this process to work this afternoon 10/22/2007 and then I tried to recall from memory the majority of the process. Please test out what I have written and make sure that it works, updating the language if necessary if it gets confusing. When you are done you can send me the edited document, and I can put it back out on the network.
Todo:
- test the documentation
- screenshots for some of the processes that might get confusing
AC 10/22/07 PM