Hi all,
this seems silly, but i am a bit confused as haven’t had this be an issue before. i downloaded data from USGS NED (they have changed the process since i last used). i requested data at 1 arc second (which is supposed to be approx 30 meters)
i set up a new grass dataset, reading the projection and datum terms from a georeferenced data file (referring to the w001001.adf file). at the ‘set default region extent and resolution’ page, the correct N/S/E/W coordinates are shown, however, the N-S & E-W resolution are set to 0.000277777777841. it would not let me change the resolution. accepted and set the region. then used r.in.gdal to import the raster (the w001001.adf file), then set the region to the new dem raster and displayed it. so all seemed to work fine. however, when i check the region settings with g.region -p, the nsres/ewres are 0:00:01.
i need this to be a 30 meter DEM, and am confused as to what the present resolution actually is, as i requested 30m (1 arc second) at download from NED.
anyone have any tips regarding this?
thank you,
Janet
Hi Janet,
Tthe data seems to be in a geographic (angular) coordinate system, which means that you won’t be able to get resolution in meters. Due to the curvature of the Earth’s surface, one arc second has a different length at different latitudes. That’s exactly what you’re seeing in your example - 0:00:01 means 0° 0" 0’. The notation is in degrees minutes seconds (DMS) rather than decimal degrees (DD), which I prefer, but however you look at it, unless you reproject the data into a projected coordinate system like UTM, etc., the resolution will be in that unit.
The short version of that long story is that your data sounds like it’s fine - 1 arc second is about 30m and so it looks like you’ve got the data you wanted and GRASS interpreted it correctly.
Best,
Daniel
–
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2012/8/8 Janet Choate <jsc.eco@gmail.com>
Hi all,
this seems silly, but i am a bit confused as haven’t had this be an issue before. i downloaded data from USGS NED (they have changed the process since i last used). i requested data at 1 arc second (which is supposed to be approx 30 meters)
i set up a new grass dataset, reading the projection and datum terms from a georeferenced data file (referring to the w001001.adf file). at the ‘set default region extent and resolution’ page, the correct N/S/E/W coordinates are shown, however, the N-S & E-W resolution are set to 0.000277777777841. it would not let me change the resolution. accepted and set the region. then used r.in.gdal to import the raster (the w001001.adf file), then set the region to the new dem raster and displayed it. so all seemed to work fine. however, when i check the region settings with g.region -p, the nsres/ewres are 0:00:01.
i need this to be a 30 meter DEM, and am confused as to what the present resolution actually is, as i requested 30m (1 arc second) at download from NED.
anyone have any tips regarding this?
thank you,
Janet
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Hi Janet,
2012/8/9 Janet Choate <jsc.eco@gmail.com>
i reprojected to utm with r.proj, that worked great. however, i’m still a bit worried about the resolution. after i reprojected to utm, the region settings now show the nsres/ewres to be approx 1.0 and not 30. i can aggregate up to 30 - but i’m worried if i do so. should i trust that it is really 1 meter? i requested 1 arc second data (30m) from NED, but it is possible i ended up with 1m. do you know if there is any way to tell if the data is really 1m or already at 30m - other than with g.region?
I’m not sure on that one. It could be that you’ve got the wrong data. However, reprojecting using r.proj takes the region settings of the target mapset. Therefore if had a resolution of 1 m beforehand, the source maps will have been resampled to 1 m while reprojecting (the default method is nearest neighbor).
I would zoom in and take a look at the data using the measuring tool. If it looks like it’s been resampled from 30m to 1m (especially using nearest neighbor), then that’s probably what happened 
There’s a great help text on reprojecting rasters in the manual of r.proj. If you’re wanting to look at it on the web, check it out here:
http://grass.osgeo.org/manuals/html70_user/r.proj.html
For some reason, when I google r.proj I get two versions of the manual. The second one with the link is the one I find more helpful because it has code in it that you can pretty much cut and paste into your GRASS environment.
Best,
Daniel