In the course of trying to figure out a specific data extraction task, I'm trying to understand the coordinate system used by GRASS. For a while, I'd been working with elevation data near Raleigh, NC, provided in the GRASS tutorial section. Search for "GRASS 7 LOCATION nc_spm_08" at:
http://grassbook.org/datasets/datasets-3rd-edition/
Meanwhile, I downloaded elevation data from USGS, using the instructions linked from their FAQ:
http://ned.usgs.gov/faq.html#DATA
(Aside: I tried to get data for a roughly 5-km square covering part of the Grand Canyon, and ended up with data for the entire CONUS instead. Given how meticulously I tried to follow the directions, I'm inclined to think their tool would've given me the entire country no matter what region I chose, but I can't confirm that.)
I can import the USGS data (srgr4810100a.tif - a 1.5GB file) into GRASS, but then the coordinates don't match with those of Raleigh - for GRASS' Raleigh data, the coordinates are around (639000,224000), while for USGS, Raleigh is showing somewhere around (1580000,1580000) - clearly nowhere close to the same ballpark.
If I approach this problem from the other direction - namely, reading what I can about GRASS' coordinate systems, I similarly get nowhere:
http://grass.osgeo.org/gdp/grass5tutor/HTML_en/c1419.html
All the above says is about what I already know, involving the difficulties in projecting a Cartesian coordinate system onto a spherical territory. It says absolutely nothing about how I'm supposed to interpret the numbers in the lower left of GRASS' GUI display when the status bar's display combobox is set to "Coordinates", or how (or even if) I'm supposed to use those numbers in other tools (such as the ever-important Region setting).
Can anyone shed light on what I'm missing? I thought both maps were supposed to include sufficient information about their frame of reference in their metatdata such that, even if they were different, GRASS could convert between them, especially since I understand both sources to be reputable.
I have roughly two weeks' worth of familiarity with geospatial analysis tools and concepts in general - mainly GRASS and GeoTools - so feel free to factor that in.
أكثر...
http://grassbook.org/datasets/datasets-3rd-edition/
Meanwhile, I downloaded elevation data from USGS, using the instructions linked from their FAQ:
http://ned.usgs.gov/faq.html#DATA
(Aside: I tried to get data for a roughly 5-km square covering part of the Grand Canyon, and ended up with data for the entire CONUS instead. Given how meticulously I tried to follow the directions, I'm inclined to think their tool would've given me the entire country no matter what region I chose, but I can't confirm that.)
I can import the USGS data (srgr4810100a.tif - a 1.5GB file) into GRASS, but then the coordinates don't match with those of Raleigh - for GRASS' Raleigh data, the coordinates are around (639000,224000), while for USGS, Raleigh is showing somewhere around (1580000,1580000) - clearly nowhere close to the same ballpark.
If I approach this problem from the other direction - namely, reading what I can about GRASS' coordinate systems, I similarly get nowhere:
http://grass.osgeo.org/gdp/grass5tutor/HTML_en/c1419.html
All the above says is about what I already know, involving the difficulties in projecting a Cartesian coordinate system onto a spherical territory. It says absolutely nothing about how I'm supposed to interpret the numbers in the lower left of GRASS' GUI display when the status bar's display combobox is set to "Coordinates", or how (or even if) I'm supposed to use those numbers in other tools (such as the ever-important Region setting).
Can anyone shed light on what I'm missing? I thought both maps were supposed to include sufficient information about their frame of reference in their metatdata such that, even if they were different, GRASS could convert between them, especially since I understand both sources to be reputable.
I have roughly two weeks' worth of familiarity with geospatial analysis tools and concepts in general - mainly GRASS and GeoTools - so feel free to factor that in.
أكثر...