[GIS] find USGS SRTM 3 index grid or what procedure should I employ to select tiles

demsrtm

I would like to download all USGS SRTM 3 tiles for Ontario, Canada from the USGS site: http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/North_America/

Where can I obtain the index that will assist me in querying the tiles I need to download. Alternatively, is there another easier way to identify the tiles?

When I was downloading the tiles for Manitoba, Canada I found a range of files then just downloaded the individual files from the site. Is there a similar range of files for Ontario?

Thanks

Best Answer

alt text

http://dds.cr.usgs.gov/srtm/version2_1/Documentation/Continent_def.gif

You can use http://www.latlontoelevation.com/ lat/lng to get the Shuttle Radar Topography Mission (SRTM) data too.

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When you downloaded the data what format did it come in? By definition, if the data is a raster format, it can be considered 3D data, as it has a x,y,z value. If you are looking to have it extruded in ArcScene then you will want to do a few of the following things.

Assuming your data is in a usable raster format (ie: GeoTiff) then you will want to do the following: - Open the Properties of the data in ArcScene and change the Symbology (ie: Standard Dev. 2 and change the statistics to From Current Display Extent - On the Base Heights Tab change the radio buttom from No elevation values from a surface to Floating on a custom surface and choose the SRTM raster file (this will make the raster have 3D visual appeal - Finally you can play with the visual rendering which is located under the Rendering tab. The Higher the rendering, the more memory it needs from your computer and graphics card.

Seamless NED Tiles – How to Programmatically Download NED Tiles

Briefly, USGS has application services (Option #1), but for some data sets it's also possible to generate direct download URLs (Option #2) to the public location of files.

Download Option #1: USGS Application Services

There's documentation here about the web services that are available: http://nimbex.cr.usgs.gov/app_services.php

If you're interested in elevation data, be aware of this:

As of July 1, 2013, National Elevation Dataset (NED) 1- and 1/3-arc-second data are distributed only through pre-packaged 1x1 degree tiles in ArcGrid, GridFloat and IMG formats. Because of their very large file sizes, NED 1/9 arc-second data is distributed only through pre-packaged 15x15 minute tiles in IMG format.

I believe the changes (no more seamless) to the web services over the past couple years are related to this.

This is legit way to download data programmatically. But if you only need some simple tiles...

Download Option #2: Tiles via easy to generate URLs

The National Map Viewer can be used for downloading all kinds of data, but of course that is not programmatic. But if you make an "order" from the National Map Viewer, you will notice that the download links it sends you are very simple to generate.

For example, let's look at Elevation data since it's one of the more complicated ones to figure out. The links in the email look like this:

http://gisdata.usgs.gov/TDDS/DownloadFile.php?TYPE=ned1g_zip&FNAME=n36w100.zip&ORIG=TNM&dlpre=1-9
http://gisdata.usgs.gov/TDDS/DownloadFile.php?TYPE=ned1g_zip&FNAME=n36w101.zip&ORIG=TNM&dlpre=2-9
http://gisdata.usgs.gov/TDDS/DownloadFile.php?TYPE=ned3g_zip&FNAME=n41w074.zip&ORIG=TNM&dlpre=1-1

The base URL is: http://gisdata.usgs.gov/TDDS/DownloadFile.php

Followed by the following arguements:

TYPE
- ned1g_zip = NED 1 arc sec, ArcGrid format
- ned1f_zip = NED 1 arc sec, GridFloat format
- ned1i_zip = NED 1 arc sec, IMG format
- ned3g_zip = NED 1/3 arc sec, GridFloat format
- ...
FNAME
- The coordinates of the upper left corner of the tile. This is the actual filename of the Zip files on their server. Example: n36w100.zip would be the tile with an upper left corner at Lat/Long 36,-100
ORIG
- TNM - Let's just pretend we're the National Map Viewer...
dlpre
- This one is not required. (If you select a large area with more than one tile, it signifies the which number tile out of the total number tiles you're downloading).

I'm thinking this would be the preferred way to do things, but you'll probably notice that DownloadFile.php is just generating the actual URL of the tile.....

So in reality, the location of the tiles look like this:

http://tdds3.cr.usgs.gov/Ortho9/ned/ned_1/grid/n38w102.zip
http://tdds3.cr.usgs.gov/Ortho9/ned/ned_13/grid/n38w102.zip
http://tdds3.cr.usgs.gov/Ortho9/ned/ned_13/float/n38w102.zip

Which again we can easily break down into

ned_1, ned_13
grid, float, img
filename of tile.zip

For Landcover:

http://tdds3.cr.usgs.gov/ortho9/nlcd/2006/landcover/conus/NLCD2006_LC_N39W072_v1.zip

This method does not work well with higher resolution data (1/9 arc second DEM, orthoimagery) because the filenames don't follow a simple pattern.

Best Answer

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