GRASS FAQ

This pages contains a growing list of Frequently Asked Questions (FAQs). Feel free to add text, "Edit" button for that. See also Category: FAQ for more articles.

Usage

1. In a site map, how to select all the points that are at the edge? - Points Edge Selection
2. How to create a buffer around polygons using v.buffer? - Vector Buffer
3. What are LOCATIONs and MAPSETs? - Location and Mapsets
4. Starting GRASS - Starting GRASS
5. ERROR: Default region not set - Error Region
6. Nothing shows in monitor window - Region Map
7. How to show labels, such as street labels on a vector map? - Labels Show
8. What is a raster MASK? - GRASS Raster Mask
9. What is an attribute? - GRASS Attributes
10. Is is possible to switch from one LOCATION to another while running GRASS? - Switch Locations
11. Changing region during operation? - Changing Region
12. Point in polygon test: - Point in Polygon
13. Georeferencing an image?
14. How do I reproject a raster or vector map? - Map Reprojection
15. How do I enjoy high quality statistic analysis in GRASS? GRASS and Rstat
16. What is the difference between pseudo-topological and real topological vect formats? - Topo Issue
17. How to to aggregate values from one polygon layer to another ? - Aggregate Values
18. How to diplay a vector (polygon) map colored by the attributes in a particular column? Vector Attribute Display
19. How to Convert vector types
20. How to Dissolve polygons

Data

1. Can I map data from an arbitrary database model to work with GRASS without having to restructure or create a new database?
2. How can I drop columns in a sqlite-database? - Sqlite Drop Column
3. How to unlock SQLite database? - SQLite unlock database

Technical

1. GRASS 6 symbols - IconSymbols
2. How do I compile/install GRASS GIS? Compile and Install

Old stuff:

1. GRASS 5.7 vector architecture - Vector Faq GRASS57
2. Does GRASS 5.7 installation override my stable GRASS5x installation? GRASS5x and 57
3. Are GRASS 5.0.x/5.3.x/5.4.x raster maps compatible with GRASS57? Yes!

Installation

GRASS 6 compilation problems

• Can't find a library even after I've built and installed it: Make sure that /usr/local/lib (or where you installed it) is listed in your /etc/ld.so.conf file and (as root) run /sbin/ldconfig.
• How do I install grass in MS Windows: - WinGRASS Installation

GRASS and Cygwin

1. How do I find the Cygwin packages to install? - See Cygwin Packages
2. I can't find pq.dll: Make sure you installed the PostgreSQL package in the Cygwin [installer].
3. I can't find pq.dll: try copy cygpq.dll to pq.dll

Errors

• In a Latitude-Longitude location I'm getting the error Illegal latitude for North when importing an unprojected raster image: Unprojected images are given a resolution of 1 pixel per line, and so any image with a height of more than 90 pixels will try and write data to an impossible latitude.

Solution: import the image into a simple XY location and georeference the image with either r.region (if bounds are simple and known) or i.points and i.rectify. An automated alternative to r.region is to create a "World File" to be used by r.in.gdal. See the GeoTIFF format help page at http://www.gdal.org.

The command r.in.gdal has a -l flag to constrain the import to valid lat/lon ranges. Another good alternative is to use gdal_translate -a_ullr to convert the dataset into a GeoTiff with reset bounds. See the MODIS wiki page for details.

Use gdalinfo to test your data set. What it sees, r.in.gdal will see.

Sometimes you get this error if the input data tries to exist a little north-of-north and south-of-south. E.g. 90.5N - 90.5S. In these cases take care with the grid-versus-cell center raster convention when resetting the bounds. In GRASS the outer edge of all the cells are given as the n,s,e,w extent, and the data value for each cell represents the value within the area of all of that ewres*nsres cell. You might use gdal_translate -srcwin or -projwin to crop away the top/bottom row of cells so that the data does not try to place itself at an impossible latitude.