Tips for Arc users

Basic tutorial for GRASS GIS

See GRASS GIS tutorial - at Geostat 2012

Working with your ArcGIS data

Importing ArcGIS Data

Grass provides convertors for importing ESRI shapefiles, e00 files, and many other GIS formats as well. The key Grass programs for importing vector formats are v.in.ogr (for ESRI shapefiles, MapInfo files, SDTS, TIGER, etc.) and v.in.e00 for e00 format.

Coordinate Reference Systems

The GRASS data structure requires that each data layer have its CRS exactly defined. This is done by maintaining data layers within a "Location". Each Location has a strictly defined projection, and datum, and all the data layers (raster and vector) within that location should already be either created in or projected to that CRS. GRASS does not do "on-the-fly" reprojection due to the problems often encountered in other GIS software with datum mismatches. So data layers in one location are not visible from other locations. However GRASS does supply a collection of tools for projecting vector and raster layers from one location to another. Use of these tools is explained in the projectionintro manual.

Vector import/export commands

• v.in.ogr - Convert OGR supported vector formats to GRASS vector format.

• v.out.ogr - Convert from the GRASS vector format to one of the supported OGR vector formats.

• v.in.e00 - Import a E00 coverage into a GRASS vector map.

Import of Shapefiles into Grass

specify the directory

v.in.ogr dsn=/home/data/navigation_files output=Tracklines layer=Ship_Tracklines

or just the .shp file name directly

v.in.ogr dsn=Ship_Tracklines.shp output=Tracklines

This is the syntax for v.in.ogr in its most basic form. The dsn parameter corresponds to the directory path of the vector you are trying to import. You can enter a full or relative path. The output paramter, aptly enough, is the name of your output Grass vector. The layer parameter is the name of the input vector, be it shapefile, MapInfo file, or what have you.

Export of Shapefiles from GRASS

Shapefiles can only hold one type of data (point, line, polygon) per shapefile. You can't export a GRASS vector map with both points and areas to a single shapefile. You'll have to do it in two passes to two different files with v.out.ogr and different "type=" parameters.

Export lines from GRASS vector map to Shapefile format (generates /tmp/testogr.shp and related files):

v.out.ogr input=multi type=line dsn=/tmp olayer=testogr 

Export areas from GRASS vector map to Shapefile format (generates /tmp/testogr.shp and related files):

v.out.ogr input=multi type=area dsn=/tmp olayer=testogr 

Export 3D lines from GRASS vector map to Shapefile format:

v.out.ogr input=lines_3d type=line dsn=/tmp olayer=testogr lco="SHPT=ARCZ"

Various vector commands

• Editing differences

Tips for Digitizing area features with attributes

Raster import/export commands

• r.in.arc - Convert an ESRI ARC/INFO ascii raster file (GRID) into a GRASS (binary) raster map layer.

• r.out.arc - Converts a GRASS raster map layer into an ESRI ARCGRID file.
  • Some tips and tricks for imporing Arc ASCII grids from external software

• r.in.gdal - Import GDAL supported raster file into a binary raster map layer.
  • Import a binary ArcInfo raster grid into GRASS:
    • See the GDAL help page for AIC
    • Often the biggest .adf file is the right one to import -- check your data values after importing even if they visually look correct!

r.in.gdal input=/path/to/map/hdr.adf output=map

• r.out.gdal - Exports GRASS raster data into various formats.
  • Export a raster as a GeoTIFF for use with ArcView 3.1:

r.out.gdal input=map output=map.tif type=Byte createopt="INTERLEAVE=PIXEL,COMPRESS=PACKBITS"

Import ArcASCII raster grid and connect to database

The trick is to convert the raster coverage to vector areas to take advantage of GRASS's rich vector attribute/database tie-ins. You can use either r.in.arc or r.in.gdal for the import. If using r.in.arc be careful to set the map type to CELL for categorical maps.

# import ArcASCII grid to UTM55S location
r.in.gdal  input=aas_z55.asc output=aas_z55
r.colors aas_z55.gdal color=random
r.info aas_z55

# convert to vector areas, using raster value for category
g.region rast=aas_z55
r.to.vect -v in=aas_z55.gdal out=aas_z55 feature=area

# get rid of empty DBF table created by r.to.vect
v.db.droptable aas_z55

# copy real DBF file into $MAPSET/dbf/ directory
eval `g.gisenv`
cp /tmp/au/aas_z55.DBF "$GISDBASE/$LOCATION_NAME/$MAPSET/dbf/aas_z55.dbf"

# connect full DBF database to vector map, use "VALUE" as the key column
v.db.connect map=aas_z55 driver=dbf table=aas_z55 key=value

done!

Import E00 raster grids

The v.in.e00 module only handles vector data. You will have to convert raster E00 data into an Arc ASCII grid before importing into GRASS with r.in.arc or r.in.gdal.

• See instructions in this mailing list post about the

"ArcExplorer Import Utility"

Various hydrology and terrain analysis commands

• Watershed modeling differences:
  • ArcInfo wants a depressionless, filled, hydrologically correct DEM.
  • r.watershed does not need such a DEM, it is happy with a raw DEM even containing depressions, being not filled, and not hydrologically correct (hence, no extra preparations needed!).
• How to Create watersheds for specific drainage points like the Arc Hydro extension
• See also Hydrological Sciences for a list of available hydrological GRASS tools
• Watershed Analysis with GRASS (when you know ESRI)

See also

• A transition from ArcGIS to open source GIS softwares
• Can ArcGIS and OSGeo4W share the same python install?

GRASS migration hints