[GRASS-user] First time GRASS user -- how to import an img dataset?

[Micha Silver]

On 09/29/2010 06:12 PM, Mario Julio Barragan Arce wrote:
> Hello. I need to learn how to use GIS and I have chosen to do it with 
> GRASS. My plan is to learn GIS/GRASS essentials during the next 12 
> months and start applying it for my 

Welcome, and good decision.
> research the 12 months after that. I have started reading the 
> documentation available and playing with the sample data set. However, 
> I would also like to start playing with a dataset for Puerto Rico (PR) 
> and I am having problems to import it. How would I go about importing 
> a new dataset?
>
> In particular, there is a landcover raster image in Erdas "img" format 
> (prgap_landcover.img) that I would like to import from here:
>
> ftp://ftp.gap.uidaho.edu/products/Puerto_Rico/GISdata/Landcover/1_Land_cover_grid/
>
> (1) What do I do with this img file? Do I first copy and paste it to 
> my GIS Data Directory? If not, how do I go about telling GRASS where 
> this file is (if I want to import it)?
>
If the file is somewhere on your computer, then just enter the path to 
it with the 'input=' parameter to r.in.gdal. Something like:
r.in.gdal in=/path/to/prgap_landcover.img out=prgap_landcover

(BTW, I think you can actually even import into GRASS straight from the 
URL...)

> (2) I read about the commands to import raster Erdas/IMG files to 
> GRASS (r.in.gdal -e in=<image.img> out=<image>). If I want to use this 
> command, should my img file be in the GIS Data Directory first as I 
> suggested in (1)? Where do I type this command? The GRASS Command Line 
> window seems to open for an existing Location.
>
> (3) When do I create the location for my data? Do I do this before 
> anything else?

YES, before anything else. This step is crucial, and sometimes confuses 
new users. You *must* have a correct LOCATION defined at the start such 
that it matches the CRS of the original data. GRASS always insists that 
all maps in a LOCATION are projected in exactly the same CRS.
 From the metadata below it looks like this file is projected in the LCC 
projection matching the EPSG code of 2866. That's NAD83(HARN) Puerto 
Rico and Virgin Islands.
You might try to first use the wizard to create a new location based on 
this EPSG code. That insures all the parameters get entered properly. 
Then, after starting GRASS in this location (also create a MAPSET - the 
GRASS way to keep maps filed in some directory which is logical for your 
work), try the r.in.gdal command.

>
> (4) When trying to create a location from the Location Wizard, I chose 
> as the method "Select coordinate system parameters from a list". I did 
> this because there is another file in the above mentioned ftp folder 
> with several parameters which seem to me the kind of information that 
> the wizard asks (see an excerpt below). From there I chose "lcc" as 
> the projection code. However, from here onwards I start having 
> problems. For instance, I don't know whether to choose "Datum with 
> associated ellipsoid" or "Ellipsoid only". Also, the wizard asks me 
> "Central Parallel", "First Standard Parallel", "Second Standard 
> Parallel", and I don't know how that relates to the two 
> "Standard_Parallel" parameters given below and where do I get the 
> other "Parallel" parameter from? I seem to be missing one. For the 
> geodetic datum I think I have to choose "nad83" and I think I will 
> choose "No transformation" at the end of the wizard.
>
> Any suggestions will be very welcome. Thank you in advance. Saludos, 
> Julio.
>
Cheers. For future questions, let us know your OS and version of GRASS. 
And posting the output of commands like g.region -p, v.info, r.info will 
often help.

Regards,
Micha
> Spatial_Reference_Information:
>    Horizontal_Coordinate_System_Definition:
>      Planar:
>        Map_Projection:
>          Map_Projection_Name: Lambert Conformal Conic
>          Lambert_Conformal_Conic:
>            Standard_Parallel: 18.033333
>            Standard_Parallel: 18.433333
>            Longitude_of_Central_Meridian: -66.433333
>            Latitude_of_Projection_Origin: 17.833333
>            False_Easting: 200000.000000
>            False_Northing: 200000.000000
>        Planar_Coordinate_Information:
>          Planar_Coordinate_Encoding_Method: row and column
>          Coordinate_Representation:
>            Abscissa_Resolution: 15
>            Ordinate_Resolution: 15
>          Planar_Distance_Units: meters
>      Geodetic_Model:
>        Horizontal_Datum_Name: North American Datum of 1983
>        Ellipsoid_Name: Geodetic Reference System 80
>        Semi-major_Axis: 6378137.000000
>        Denominator_of_Flattening_Ratio: 298.257222
>    
>
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-- 
Micha Silver
Arava Development Co. +972-52-3665918
http://surfaces.co.il


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