GRASS GIS for ArcGIS users - Revision history

⚠️Wenzeslaus: /* Projections */ some tools dealing with terrain require projection info in ArcGIS as an input

2016-02-26T17:52:17Z

Projections: some tools dealing with terrain require projection info in ArcGIS as an input

← Older revision		Revision as of 17:52, 26 February 2016
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	When data are in different projection they can be imported into GRASS GIS using {{cmd\|r.import}} and {{cmd\|v.import}} modules. When the data are already in another GRASS Location which has different projection than we want, we can create a new Location with the desired projection and bring the data there using {{cmd\|r.proj}} and {{cmd\|v.proj}}.		When data are in different projection they can be imported into GRASS GIS using {{cmd\|r.import}} and {{cmd\|v.import}} modules. When the data are already in another GRASS Location which has different projection than we want, we can create a new Location with the desired projection and bring the data there using {{cmd\|r.proj}} and {{cmd\|v.proj}}.

			Once we import our data into GRASS GIS database with the right projection, we don't have to deal with the projection conversions and or inconsistencies and we can focus on the analysis. GRASS GIS analytical modules won't ask us to provide projection for the output dataset which is a common thing for some ArcGIS tools.

	== Data, databases and file formats ==		== Data, databases and file formats ==

⚠️Wenzeslaus: /* See also */ add external link

2016-02-09T14:59:05Z

⚠️Micha: Further details in the Flow Direction section

2016-01-29T14:36:55Z

Further details in the Flow Direction section

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 == Flow Directions ==
-* [http://www.surfaces.co.il/flow-direction-from-grass-to-arcgis/  Flow Directions from GRASS to ArcGIS]
+Like the ArcGIS FLOWDIRECTION function, GRASS can also create a flow direction raster, giving each cell an integer value representing one of the eight directions of flow out of the cell. Flow direction in both programs is determined by finding which of the surrounding cells has the lowest elevation value. However, the values that each software uses are different. In ArcGIS, the flow directions are numbered from east clockwise, and each direction is a power of 2 higher than the previous. Thus, when the flow direction is east out of a cell, that cell is given the value 1. When south-east the value is 2. Then, a cell with flow direction going south gets the value 4, and so on until north-east, which becomes 128. (Details of this procedure are presented in [http://resources.arcgis.com/en/help/main/10.1/index.html#//009z00000052000000 the ArcGIS Resources website])
 == See also ==

⚠️Wenzeslaus: /* Further notes */ mention vector algebra

2016-01-22T21:01:09Z

Further notes: mention vector algebra

← Older revision		Revision as of 21:01, 22 January 2016
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	=== Further notes ===		=== Further notes ===

	Note that there are different algebras in GRASS GIS, namely it is 3D raster algebra which the same as the standard 2D one with few differences to accommodate 3D rasters and then it is temporal algebra which contains a lot of additional syntax to work with spatio-temporal data.		Note that there are different algebras in GRASS GIS, namely it is 3D raster algebra which the same as the standard 2D one with few differences to accommodate 3D rasters and then it is temporal algebra which contains a lot of additional syntax to work with spatio-temporal data. There is also a module supporting vector map algebra in GRASS GIS Addons repository.

	== Python interface ==		== Python interface ==

⚠️Wenzeslaus: /* Projections */ update info about r.import and fix typos

2016-01-04T04:00:45Z

Projections: update info about r.import and fix typos

← Older revision		Revision as of 04:00, 4 January 2016
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	== Projections ==		== Projections ==

	ArcGIS supports on-the-fly projection of spatial data while GRASS GIS considers this as a bad practice and requires users to have consistent projection ~~informaion~~ for all the data entering the analysis. This is one of the ways how GRASS GIS helps user to manage the integrity of their data. The other ways are keeping the geospatial data organized and requiring the data to be in a one format, e.g. to store vector data in the GRASS topological format. However, for raster data there is for example the {{cmd\|r.external}} module which allows to link (rather than import) and use data in different formats as long as they are in the same projection.		ArcGIS supports on-the-fly projection of spatial data while GRASS GIS considers this as a bad practice and requires users to have consistent projection information for all the data entering the analysis. This is one of the ways how GRASS GIS helps user to manage the integrity of their data. The other ways are keeping the geospatial data organized and requiring the data to be in a one format, e.g. to store vector data in the GRASS topological format. However, for raster data there is for example the {{cmd\|r.external}} module which allows to link (rather than import) and use data in different formats as long as they are in the same projection.

	When data are in different projection they can be imported into GRASS GIS using {{cmd\|r.import~~\|version=71~~}} and {{cmd\|v.import~~\|version=71~~}} modules ~~which are available in GRASS GIS Addons for GRASS GIS 7.0 and will be included in version 7.1~~. When the data are already in a GRASS Location ~~but they have~~ different projection ~~then~~ we want, we can create a new Location with the desired projection and ~~import~~ the data there using {{cmd\|r.proj}} and {{cmd\|v.proj}}.		When data are in different projection they can be imported into GRASS GIS using {{cmd\|r.import}} and {{cmd\|v.import}} modules. When the data are already in another GRASS Location which has different projection than we want, we can create a new Location with the desired projection and bring the data there using {{cmd\|r.proj}} and {{cmd\|v.proj}}.

	== Data, databases and file formats ==		== Data, databases and file formats ==

Neteler: +Flow Directions

2015-10-06T07:48:20Z

+Flow Directions

← Older revision		Revision as of 07:48, 6 October 2015
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	== Cost analysis ==		== Cost analysis ==
	Cost surface is defined differently in GRASS GIS and ArcGIS. Cost in GRASS GIS is cost to ''cross a cell'' while in ArcGIS it is cost to ''cross one map unit''. So for example we define cost as time and we have 30 m cells and speed is 5 m/s. Then the cost value of the cell is 6 s in GRASS GIS and 0.2 s in ArcGIS.		Cost surface is defined differently in GRASS GIS and ArcGIS. Cost in GRASS GIS is cost to ''cross a cell'' while in ArcGIS it is cost to ''cross one map unit''. So for example we define cost as time and we have 30 m cells and speed is 5 m/s. Then the cost value of the cell is 6 s in GRASS GIS and 0.2 s in ArcGIS.

			== Flow Directions ==
			* [http://www.surfaces.co.il/flow-direction-from-grass-to-arcgis/ Flow Directions from GRASS to ArcGIS]

	== See also ==		== See also ==

⚠️Wenzeslaus: intro sentence about page purpose

2015-10-03T19:25:48Z

intro sentence about page purpose

← Older revision		Revision as of 19:25, 3 October 2015
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			''This is a guide for people who want to migrate from ArcGIS to GRASS GIS or just want to learn GRASS GIS and are already familiar with ArcGIS.''

	== Projections ==		== Projections ==

⚠️Wenzeslaus: /* Data, databases and file formats */ mention File Geodatabase and Personal Geodatabase

2015-08-31T01:26:21Z

Data, databases and file formats: mention File Geodatabase and Personal Geodatabase

← Older revision		Revision as of 01:26, 31 August 2015
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	Users can select different strategies how to use the database directory, Locations and Mapsets. Often users have one GRASS GIS database directory called <code>grassdata</code>. Then every Location is a bigger project or a area of interest. Mapsets are then smaller projects or individual tasks. PERMANENT Mapset is used to keep common data in one place.		Users can select different strategies how to use the database directory, Locations and Mapsets. Often users have one GRASS GIS database directory called <code>grassdata</code>. Then every Location is a bigger project or a area of interest. Mapsets are then smaller projects or individual tasks. PERMANENT Mapset is used to keep common data in one place.

			GRASS GIS Location or Mapset are in certain way similar to File Geodatabase or Personal Geodatabase as they store all data inside them in a given format.

	== Cartography ==		== Cartography ==

Neteler: some minor wording fixes

2015-08-25T22:52:19Z

some minor wording fixes

← Older revision		Revision as of 22:52, 25 August 2015
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	== Projections ==		== Projections ==

	ArcGIS supports on-the-fly projection of spatial data while GRASS GIS considers this as a bad practice and requires ~~user~~ to have consistent projection for all the data entering the analysis. This is one of the ways how GRASS GIS helps user to manage integrity of ~~the~~ data. The other ways are keeping the geospatial data organized and requiring the data to be in a one format, e.g. to store vector data in the GRASS topological format. However, there is for example, {{cmd\|r.external}} module which allows to link and use data in different formats as long as they are in the same projection.		ArcGIS supports on-the-fly projection of spatial data while GRASS GIS considers this as a bad practice and requires users to have consistent projection informaion for all the data entering the analysis. This is one of the ways how GRASS GIS helps user to manage the integrity of their data. The other ways are keeping the geospatial data organized and requiring the data to be in a one format, e.g. to store vector data in the GRASS topological format. However, for raster data there is for example the {{cmd\|r.external}} module which allows to link (rather than import) and use data in different formats as long as they are in the same projection.

	When data are in different projection they can be imported into GRASS GIS using {{cmd\|r.import\|version=71}} and {{cmd\|v.import\|version=71}} modules which are available in GRASS GIS Addons for GRASS GIS 7.0 and will be included in version 7.1. When the data are already in a GRASS Location but they have different projection then we want, we can create a new Location with desired projection and import the data there using {{cmd\|r.proj}} and {{cmd\|v.proj}}.		When data are in different projection they can be imported into GRASS GIS using {{cmd\|r.import\|version=71}} and {{cmd\|v.import\|version=71}} modules which are available in GRASS GIS Addons for GRASS GIS 7.0 and will be included in version 7.1. When the data are already in a GRASS Location but they have different projection then we want, we can create a new Location with the desired projection and import the data there using {{cmd\|r.proj}} and {{cmd\|v.proj}}.

	== Data, databases and file formats ==		== Data, databases and file formats ==

	In ArcGIS, users often have data in different directories on disk. Basically all geospatial data for GRASS GIS are in GRASS GIS database. This database is divided into Locations and Locations are further divided into Mapsets. When GRASS GIS is started it connects to a database, Location and Mapset specified by the user.		In ArcGIS, users often have data in different directories on disk. Basically all geospatial data for GRASS GIS are in the GRASS GIS database (a folder on a disk or network drive). This database is divided into ''Locations'' and Locations are further divided into ''Mapsets''. When GRASS GIS is started, it connects to database, Location and Mapset as specified by the user.

	By this GRASS GIS ~~sets~~ a specific system to organize the data. The system is designed to accommodate large projects and even multiple users but also ~~to be~~ useful for individuals who just want to keep their data organized. First, data must be in one directory called GRASS GIS database directory. You can have one or more of these directories on your disk. This directory contains GRASS Locations. All data in one Location have the same projection (coordinate system, datum). Location is a directory which contains GRASS Mapsets. Mapset contains raster and vector maps (layers) and other geospatial data.		By this GRASS GIS is able to apply a specific system to organize the data. The system is designed to accommodate large projects and even multiple users in a Location but it is also useful for individuals who just want to keep their data well organized. First, data must be in one directory called GRASS GIS database directory. You can have one or more of these directories on your disk. This directory contains GRASS Locations. All data in one Location have the same projection (coordinate system, datum). A Location is a directory which contains GRASS Mapsets. A Mapset contains raster and vector maps (layers) and other geospatial data.

	To avoid unwanted changes in the existing data, GRASS GIS modules ~~can create~~, ~~modify~~, change, or ~~delete a~~ data ~~only in~~ the current Mapset. When needed, the data can be copied into the current Mapset using {{cmd\|g.copy}}, for example when one needs to change attributes of a vector map.		To avoid unwanted changes in the existing data, GRASS GIS modules handle the creation, modification, change, copying or deletion of those data stored the current Mapset (i.e., in the actually used session). When needed, the data can be copied from other Mapsets into the current Mapset using {{cmd\|g.copy}}, for example when one needs to change attributes of a vector map not belonging to him-/herself.

	However, the data from the other Mapsets are available for reading, so you can view and use them in the analysis. By default, you see data only in the current Mapset and in a special Mapset called PERMANENT which must be present in every Location. If desired, you can view the data from other Mapsets (in the current Location) using Settings > GRASS working environment > Mapset access or using module {{cmd\|g.mapsets}}. Data in other Locations are accessible only if you start GRASS GIS in the given Location. Accessing data from multiple Location wouldn't be safe because the data in different Locations are (or can be) in different projections (while all Mapsets in one Location contains data in one particular projection).		However, the data from the other Mapsets are available for reading, so you can view and use them in the analysis. By default, you see data only in the current Mapset and in a special Mapset called PERMANENT which must be present in every Location. If desired, you can view the data from other Mapsets (in the current Location) using Settings > GRASS working environment > Mapset access or using module {{cmd\|g.mapsets}}. Data in other Locations are accessible only if you start GRASS GIS in the given Location. Accessing data from multiple Location wouldn't be safe because the data in different Locations are (or can be) in different projections (while all Mapsets in one Location contains data in one particular projection).

	You can link ~~the~~ data from different sources such as GeoTIFF or PostGIS database.		You can also link data from different sources such as GeoTIFF or PostGIS database.

	The system used in GRASS GIS results in a state when it is always defined where the new data will be created (the current Mapset). All data are in one projection and in consistent formats. The result is that once the data are imported, one can focus on analysis, not data formats.		The system used in GRASS GIS results in a state when it is always defined where the new data will be created (the current Mapset). All data are in one projection and in consistent formats. The result is that once the data are imported, one can focus on analysis, not data formats.
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	opens a new window which looks like ''Map Display'' in the main GUI but it is mostly controlled by display modules (<code>d.*</code>) such as aforementioned {{cmd\|d.rast}}. Additionally, {{cmd\|d.mon}} can be used to create images directly, for example the following series of commands will create a PNG image of elevation raster map with streams vector on top of them:		opens a new window which looks like ''Map Display'' in the main GUI but it is mostly controlled by display modules (<code>d.*</code>) such as aforementioned {{cmd\|d.rast}}. Additionally, {{cmd\|d.mon}} can be used to create images directly, for example the following series of commands will create a PNG image of elevation raster map with streams vector on top of them:

			<source lang="bash">
	g.region raster=elevation		g.region raster=elevation
	d.mon start=cairo output=map.png		d.mon start=cairo output=map.png
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	d.vect map=streams		d.vect map=streams
	d.mon stop=cairo		d.mon stop=cairo
			</source>

	This can be rewritten to Python as follows:		This can be rewritten to Python as follows:

	<source lang=~~Python~~>		<source lang="python">
	import grass.script as script		import grass.script as script
	gscript.run_command('g.region', raster='elevation')		gscript.run_command('g.region', raster='elevation')

⚠️Wenzeslaus: /* Cartography */ add d.frame

2015-08-24T14:20:33Z

Cartography: add d.frame

← Older revision		Revision as of 14:20, 24 August 2015
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	gscript.run_command('d.mon', stop='cairo')		gscript.run_command('d.mon', stop='cairo')
	</source>		</source>

			When working in command line or a script, one can also take an advantage of {{cmd\|d.frame}} module which makes layout of different maps or map elements easier.

	Even when not using Python or command line one can still take an advantage of the command line interface because the commands are shown in the GUI, can be copied and then saved or shared for later use. For example, one can set a style in GUI, copy the command and get something like the following line:		Even when not using Python or command line one can still take an advantage of the command line interface because the commands are shown in the GUI, can be copied and then saved or shared for later use. For example, one can set a style in GUI, copy the command and get something like the following line:

← Older revision		Revision as of 14:59, 9 February 2016
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	* [[GRASS migration hints]]		* [[GRASS migration hints]]
	* [[Performance comparison GRASS vs. ArcGIS]]		* [[Performance comparison GRASS vs. ArcGIS]]

			== External links ==

			* [http://gracilis.carleton.ca/CUOSGwiki/index.php/Comparative_Analysis_of_Cost_Path_Analysis_In_Grass_GIS_and_ArcMap Comparative Analysis of Cost Path Analysis In Grass GIS and ArcMap] at Carleton University Open Source GIS

	[[Category: Documentation]]		[[Category: Documentation]]
	[[Category: FAQ]]		[[Category: FAQ]]
	[[Category:ArcGIS]]		[[Category:ArcGIS]]