[GRASSLIST:6300] Re: California Albers and false northing

[Dylan Beaudette]

On Wednesday 30 March 2005 02:30 pm, Tim De Chant wrote:
> I'm currently working on getting Landsat images for the Bay Area into
> GRASS 6.1cvs (Mac OS X) but I keep running into issues with the
> projection.  I've searched the list and came up with one solution,
> which was to manually edit the PROJ_INFO file into this:
>
> name: Albers Equal Area
> proj: aea
> ellps: clark66
> a: 6378206.4000000004
> es: 0.0067686580
> lat_0: 0.0000000000
> lat_1: 34.0000000000
> lat_2: 40.5000000000
> lon_0: -120.0000000000
> y_0: -4000000
>

Tim, 

did you get this data from gis.ca.gov -- i.e. to old Teal Data Center GIS 
warehouse?

I have had a couple of issues working with this data, but first here are the 
projection parameters that are specified on their website:
http://drg.casil.ucdavis.edu/projection.html

CALIFORNIA ALBERS PROJECTION:
projection                  albers
units                       meters
1st standard parallel       34 00 00
2nd standard parallel       40 30 00
Central Meridian            -120 00 00
Latitude of Origin          00 00 00
False easting (meters)      0
False northing (meters)     -4000000
Datum                       NAD27
Sheroid                     Clarke 1866

...or in PROJ4 lingo : 
+proj=aea
+lat_1=34.00
+lat_2=40.50
+lat_0=0.00
+lon_0=-120.00
+x_0=0.000 
+y_0=-4000000.000 
+ellps=clrk66 
+units=m  


...in summary it is a modified AEA projection, based on the Clarke 1866 
Elipsoid or NAD27 Datum.


i have had best success reprojecting this data into UTM with gdalwarp:

gdalwarp -tps -e 0.0 -t_srs '+proj=UTM +zone=10 + datum=NAD83' 
original_landsat_geotiff new_utm_geotiff

(see the gdalwarp man page for more details on output control. )

i then use r.in.gdal to import the UTM geotiff into GRASS.


on a side note, i am actively using a large part of the data on gis.ca.gov, 
and it is always a little bit annoying to find out that most of the data is 
in UTM Z10 or Z11 NAD83, while the LANDSAT and DRG are in CA_Albers with 
NAD27 datum...


-- 
Dylan Beaudette
Soils and Biogeochemistry Graduate Group
University of California at Davis
530.754.7341