GIS Concepts
Geodesy (Projections and Datums)
Background material
- An introduction to Geodesy from NOAA: http://oceanservice.noaa.gov/education/kits/geodesy/welcome.html
- Wikipedia's Geodesy entry: http://en.wikipedia.org/wiki/Geodesy
- Wikipedia's GIS entry: http://en.wikipedia.org/wiki/GIS
- NGA Geodesy and Geophysics publications: http://earth-info.nima.mil/GandG/pubs.html
- An introduction to geodetic datums by Peter Dana: http://www.colorado.edu/geography/gcraft/notes/datum/datum.html
- Map projection concepts by Carlos Furuti: http://www.progonos.com/furuti/MapProj/CartIndex/cartIndex.html
Map projections
- UTM Zones: http://www.dmap.co.uk/utmworld.htm
- EPSG Codes: http://www.epsg.org
How GRASS deals with geodetics
- modified use of the PROJ.4 library: http://proj.maptools.org
G60:g.proj help page (GRASS 6.0)
G60:g.setproj help page (GRASS 6.0)
G60:r.proj help page (GRASS 6.0)
G60:v.proj help page (GRASS 6.0)
GIS Data types
Raster Data
Data which occurs in a regularly spaced grid. e.g. a satellite image or digital terrain map.
Region settings determine the spatial extent and resolution of the grid. (see G60:g.region)
http://grass.itc.it/grass61/manuals/html61_user/rasterintro.html
3D Raster Data (Voxel)
A stack of 2D raster maps.
http://grass.itc.it/grass61/manuals/html61_user/raster3dintro.html
G61:raster3dintro
Vector Data
Data which occurs as a series of coordinates. e.g. a GPS position or coastline map.
May be a point, line, area, etc in either 2D or 3D space.
Generally independent of region settings.
http://grass.itc.it/grass61/manuals/html61_user/vectorintro.html
Imagery Data
Pixelated photographic or satellite images, often imported from a
[GeoTIFF] or PNG image file.
As far as the GIS is concerned this is just another raster map, but there are several modules specially tailored for rectification and processing common imagery types. e.g. ortho-photos or multi-channel LANDSAT data.
http://grass.itc.it/grass61/manuals/html61_user/imageryintro.html
Site Data
Old versions of GRASS (5 and earlier) treated point data separate to line and polygon data. GRASS 6 classes all vector data features the same. Convert old sites file data into GRASS 6 vector format with the G60:v.in.sites or G60:v.in.sites.all modules.
Conversions between data types
The following table is intended to catalog transformations from one type of data to another:
[table is currently incomplete!]
| From \ To | Raster | 3D Raster | Vector | Imagery | Site |
|---|---|---|---|---|---|
| Raster | r.mapcalc | v.sample, r.volume, r.to.vect | X | ||
| 3D Raster | r3.mapcalc | X | |||
| Vector | v.to.rast, v.surf.rst, v.surf.idw | v.vol.rst, v.vol.idw | X | ||
| Imagery | X | ||||
| Site | v.in.sites | X | |||
| Data | r.in.* | r3.in.* | X |
How a GRASS project is organized
GRASS data is stored in a three level structure, the database, location and mapset. These can be found in a series of nested directories on the user's computer. All three must exist and are set at GRASS startup time.
The Database
The directory in which all GIS data is to be stored.
e.g. ~/grassdata/
The Location
A "location" is a GRASS project consisting of an area, projection definition (or unprojected), a grouping of mapsets, all with the same projection settings.
e.g. world_lat_lon, utm_zone_59, or west_coast
A "location" contains one or many "mapsets".
The Mapset
A "mapset" contains map(s), it is a subdirectory of a "location". Conceptually, if several mapsets are used in a location, they may be assigned to different users (each has one or several own mapsets to work in and cannot modify thos of other users), and/or it they are used to organize a project ("location") by subareas or subprojects. There are no specific organizational limitations.
There is always a PERMANENT mapset which is readable from all other mapsets within the same location. Read access to maps in other mapsets is managed with the 'g.mapsets' command or by adding the "@" symbol and mapset name (e.g. map@othermapset).
How the Open Source software development model works
GRASS differs from many other GIS software used in the professional world in that it is developed and distributed by users for users; mostly on a volunteer basis, in the open, and is given away for free.
Emphasis is placed on interoperability and unlimited access to data as well as software flexibility and evolution rate (both added features and bug minimization).
Free can have many meanings, as the links below illustrate, and within a project there is often a spectrum of philosophies and goals amongst developers. But it works - Free Software has revolutionized many sectors of the computing world over the last few years and continues to do so today.
- Philosophy of the Free Software Movement: http://www.gnu.org/philosophy/philosophy.html
- The Open Source Initiative has a good explanation: http://www.opensource.org
- The Free Software Foundation: http://www.fsf.org
- The GNU General Public License: http://www.gnu.org/copyleft/gpl.html
Raster GIS Analysis
Simple Raster Math
Sometimes when analyzing the relationship between two or more raster data sets, a relatively simple mathematical approach is best. One example using the r.mapcalc tool would be to look at changes between two raster data sets. By subtracting the values in these two data sets you can assume that resulting cells with a positive value have a possitive change and those with a negative value have negative change. If the cell values have a zero value then there would be no change.
GRASS comes bundled with the G61:r.mapcalc tool as well as the G61:d.m GUI interface for the tool accessable using the r.mapcalculator command. This GUI allows the user to easily assign raster sets to the variables used in the formulas and easily create mathematical strings that will result in a new raster data set containing the results.