QUOTE(ctempire @ Jan 29 2010, 10:21 PM)
How do I get the best dem available for free? I'm tired of using the 1:25000 dem available.
This is a procedure that I originally wrote for the UKTrainSim folks who came to DEMEX late (see "old codger's" link above). Edited to fit the USA...1) If you have not registered DEMEX
Before you start, in the RGE, using cursor position, identify the Lat/Long extents of the download that you wish to have. Add a tenth of a degree or two all around if you're uncertain.
If you use this procedure, you don't even have to wait for the map to load. From here, http://seamless.usgs.gov/index.php
, just click on the United States Data area and proceed as follows. This seems to always work, no zooming required or waiting for the map.
At the Seamless page, on the right side under "Layer Extent", For the USA select the Elevation that you want (as in NED 1/3 arc sec for 10 meter resolution or NED 1 arc sec for 30 m). You'll see a colored overlay of the DEM coverage.
On the left side under "Downloads", select the middle button on the top row for a download by Lat/Long coordinates. This will let you enter your Lat/Long coordinates directly. When the new window pops up, click on "Switch to decimal degrees", and then enter your Lat/Long coordinates.
At the "Request Summary Page", click on "Modify Data Request" and scroll down to make sure that the set you selected is ticked (either National Elevation Dataset (NED) 1 Arc Second or National Elevation Dataset (NED) 1/3 Arc Second).
Change the data format to GeoTIFF.
Scroll down to save changes and return to the Summary Page. Press the Download button. Unzip the archive, select the route in DEMEX, then point DEMEX to the *.tif file. That's it...2) If you have registered DEMEX
Install the Open Beta "demexbeta02.zip" for DEMEX from, http://www.digital-rails.com/files/
At "Route" on the menu bar, choose "Select route...". After the quad tree is displayed, at "DEM Management" on the menu bar, choose "Seamless NED 1 sec " or "Seamless NED 1/3 sec" (for the USA).
At "DEM Management" on the menu bar, choose "Retrieve DEMs", and you will bypass the USGS site "front end" and directly have the "Request Summary Page". This download is exactly tailored to the needs of your quad tree.
With registered Demex, here's what really happens for std tiles after you've selected "Seamless NED 1/3 sec" (10m) and then "Retrieve DEMs". You almost instantly get a pop-up USGS download page that's the start of a simple jigsaw puzzle that has rectangular parts. Depending on the shape of your quadtree, there may be one or more downloads on the first USGS page.
At the same time, a little pop-up window appears in Demex. I never press OK on that pop-up until I have safely obtained the downloads from first USGS page. When you do press OK, you'll get the next USGS download page. Continue this repetitive process until Demex tells you that you have all of the DEM's.
You're not forced to use the download folder path that Demex suggests in the little pop-up window. Since the GeoTiff file names are an 8 digit numerical and you can't tell where in the Universe they fit from the number alone, I use a sub-folder under NED10 (or NED 30 for DM's) that is named for the route. Once all of the zips are collected, unzip then so that Demex can get at the *.tif files. I always tick Delete Buffers, Fill Seams and Retain Data first. In Demex, navigate to your download folder and begin. At the File menu use Open DEM...
for the first, then Merge Dem...
to assemble the rest. Remainder of the procedure is as above.
As a side note, the DEMEX Beta can take over the former duties of the RGE in terms of adding or deleting tiles in the quad tree. You actually need only one standard tile present that was generated by the RGE to get started. DEMEX can do everything else, including the distant mountains quad tree.