GRASS 6 Tutorial/Vector data management

Vector data management

Overview

The native GRASS vector format provides:

• Vector topology
• m:n mapping of geometry features to attributes
• Vector layers
• OGC Simple Features <-> Topological Vector Conversion
• Database Management system (DBMS)
• SQLite, PostgreSQL + PostGIS, MySQL, ODBC (, DBF)

The vector geometry types supported in GRASS includes:

• Point
• Centroid
• Line
• Boundary
• Area (Boundaries + Centroid)
• Face (3D Area)
• [Kernel (3D Centroid)]
• [Volumes (Faces + Kernel)]

All types are true 3D: x,y,z

The examples that follow in this section on vector data management are based on the datasets available here

Import/Export of Vector data

The GRASS modules v.in.* and v.out.* handle the importing and exporting of Vector datasets. v.in.ogr is widely used and has simplified the import of vector data; OGR itself support a wide range of formats. v.out.ogr always exports the entire vector map

Importing an ESRI Shapefile

The following code snippet is a simple usage of v.in.ogr, which imports an ESRI Shapefile into the native vector format:

v.in.ogr dsn=boundary_county.shp out=boundary_county 

However, v.in.ogr has many options to import subsets (defined spatially or by SQL) and vector types

The same can be achieved using the GRASS Graphical Interface. The Vector import interface can be accessed through the vector layer manager:

You can easily import a subset of your data by specifying the spatial extent or an SQL query defined within the where command line argument:

v.in.ogr --o dsn=./ output=subset layer=boundary_county where="NAME = 'DUPLIN' OR NAME = 'WAKE'"

 +----------------------------------------------------------------------------+
| Layer:           subset                                                    |
| Mapset:          PERMANENT                                                 |
| Location:        nc_spm_08_geostat                                         |
| Database:        /home/daniel/GRASSWiki/data                               |
| Title:                                                                     |
| Map scale:       1:1                                                       |
| Map format:      native                                                    |
| Name of creator: daniel                                                    |
| Organization:                                                              |
| Source date:     Wed Nov  7 16:10:01 2012                                  |
|----------------------------------------------------------------------------|
|   Type of Map:  vector (level: 2)                                          |
|                                                                            |
|   Number of points:       0               Number of areas:      2          |
|   Number of lines:        0               Number of islands:    2          |
|   Number of boundaries:   2               Number of faces:      0          |
|   Number of centroids:    2               Number of kernels:    0          |
|                                                                            |
|   Map is 3D:              No                                               |
|   Number of dblinks:      1                                                |
|                                                                            |
|         Projection: Lambert Conformal Conic                                |
|               N:   258121.97732598    S:   106860.16001618                 |
|               E:   732841.78315847    W:   610047.98051883                 |
|                                                                            |
|   Digitization threshold: 0                                                |
|   Comments:                                                                |
|                                                                            |
+----------------------------------------------------------------------------+

Virtual link an ESRI Shapefile

If you prefer not to import your vector datasets into the native GRASS vector format, you can create a virtual link to it using v.external

This can be done as follows:

v.external dsn=./ output=boundary_county layer=boundary_county
Building topology for vector map <boundary_county>...
Number of nodes: 1707
Number of primitives: 1981
Number of points: 0
Number of lines: 0
Number of boundaries: 1055
Number of centroids: 926
Number of areas: 1055 
Number of isles: 1055
Number of areas without centroid: 129

Import an XY/XYZ ASCII file

You can easily generate a GRASS point vector file from a text file containing X,Y coordinates.

642934|95324
675695|139878
623277|158224
716318|138568
742527|164776
735975|196227
755631|200158

Pipe the coordinates to the module v.in.ascii

cat coords.txt | v.in.ascii out=points

Add the categories to the vector points to facilitate attribute joins:

v.category in=points out=points_final op=add
v.category points_final op=report

You can replicate the above procedure for three dimensional data (XYZ), by modifying the v.in.ascii argument list

You can easily generate a GRASS point vector file from a text file containing X,Y coordinates.

642934|95324|212
675695|139878|233
623277|158224|211
716318|138568|219
742527|164776|240
735975|196227|221
755631|200158|210

cat coords.txt | v.in.ascii -z out=points

Export a Vector Dataset

Using the module v.out.ogr it is possible to export native GRASS Vector data to a range of formats:

  ESRI_Shapefile,MapInfo_File,TIGER,S57,DGN,Memory,BNA,CSV,GML,GPX,KML,GeoJSON,GMT,SQLite,ODBC,MSSQLSpatial,
  PostgreSQL,MySQL,PCIDSK,DXF,Geoconcept,GeoRSS,GPSTrackMaker,PGDump,GPSBabel,

Introductory information on vector data processing is available from the help manual.

Grass Six Tutorial Default Settings

      -  Default settings for vector geometry;
         for vector attributes; for db.* modules

Grass Six Tutorial Geometry Management

http://grass.osgeo.org/grass57/tutorial/geom_storage.html

       -  General notes on Geometry 
         management; Managing the default settings; 
         GRASS vector architecture; Geometry stored in native format;
         Geometry stored in SHAPE file; 
         Import/export of vector data Geometry;
         Generating vector geometry from various sources

Grass Six Tutorial Attribute Management

http://grass.osgeo.org/grass57/tutorial/attrib_storage.html

       - General notes on Attribute 
         management; Managing the default settings; Examples;
         Database Schema