GRASS GIS - Introduction

Transcription

1 GRASS GIS - Introduction

2 What is a GIS A system for managing geographic data. Information about the shapes of objects. Information about attributes of those objects. Spatial variation of measurements across space without reference to specific boundaries continuous fields of measurements. A system for analyzing and modeling spatial relationships. Spatial statistics Aggregation and reclassification of observation data Statistical or process model development and implementation 2

3 What is a GIS (cont.) A system for presenting geographic information graphically and in summary Maps Tables Graphs 2D and 3D animations 3

4 Elements of the GIS Process Data Acquisition Preprocessing Data Management Manipulation and Analysis Product Generation Automation of the above process steps 4

5 Process - Data Acquisition Data acquisition is often the most expensive and time consuming part of any GIS project It may require: Purchasing data from commercial vendors Acquisition of data through field collection Aerial photography Satellite imagery GPS Ground survey Manual digitizing, either with a digitizing tablet or onscreen Download from government or other sources 5

6 Process - Preprocessing Data preprocessing is required to modify obtained data for appropriate use in the GIS. This may include Reprojection Reformatting/Conversion Subsetting Checking for errors Orthorectification 6

7 Process - Data Management Data management consists of a broad range of activities within the GIS that relate to the organization, processing, and storage of geospatial data. Activities included in data management include: Importation of external data Conversion from one data type to another within the GIS Movement of data from one GIS workspace to another Building topology Reprojecting data Joining attribute data with spatial data 7

8 Process - Manipulation and Analysis Manipulation and analysis within the GIS might be seen to overlap with management, with the primary distinction being that manipulation and analysis results in: New datasets based upon other datasets. For example: Slope and aspect derived from a digital elevation model The results of a model run that combines geospatial data and predefined processes Reclassification of data into new representations of those data Information about datasets as opposed to the datasets themselves (e.g. statistical information) 8

9 Process - Product Generation Product generation encompasses the set of activities that result in end products coming out of the GIS (some of which overlap with categories already discussed). These products include: Hard copy maps Digital representations of GIS data (e.g. TIFF, JPEG, GIF, and PNG files representing mapped data) Tabular summary data derived from geospatial data or processes employed in the GIS (e.g. model output, or summary statistical data) Exported geospatial data Exported databases 9

10 Process - Automation Automation is not a process step in itself, but instead represents the efficient combination of multiple processing steps into a single program that can be repeated with modification. The following processes often benefit from automation: Data import and management: import, reclassification, color definition, conversion Analysis automation: dataset reclassification, execution of model steps, presentation of analysis results Visualization: definition of the visualization environment, execution of individual visualization commands, reformatting of generated output. 10

11 What is GRASS GRASS GIS (Geographic Resources Analysis Support System) is an Open Source GIS application licensed under the GNU General Public License (GPL). It supports the management, visualization, and analysis of 2D/3D vector and raster data. GRASS Factoids: Originally developed by the U.S. Army Construction Engineering Research Laboratories (USA-CERL) between 1982 and 1995 Open Sourced in 1997 Over 1 million lines of code, developed by an international team of contributing programmers and organizations. Over 350 geoprocessing functions Currently at version

12 GRASS Characteristics GRASS has evolved over the last several years to provide a combination of an available GUI and a complementary command prompt from which all functions can be run. The command prompt is tightly integrated with the corresponding command prompt of the underlying operating system. The graphical user interface is generally rendered in a manner consistent with the parent OS (Windows is an exception here). The command/functional approach to geoprocess execution facilitates automation both within and outside GRASS through standard scripting languages (shell scripts, python, perl, etc.) 12

13 GUI vs. Command Prompt The execution of commands may often be accomplished via both the command prompt and via the system s graphical user interface (GUI). Pros and Cons of GUIs The GUI often provides access to complex functionality that would require the input of long commands. GUIs are very difficult to script, making it hard to develop automated and replicable processes and procedures. Some GUIs do not provide access to all of the options available through the same command executed from the command prompt. 13

14 GUI vs. Command Prompt (cont.) Pros and Cons of working at the command prompt Typing commands at the command prompt introduces the possibility of making errors in command syntax resulting in failure to execute a command or the execution of a command in an unexpected manner. Commands executed from the command prompt are easily integrated into automated processes and procedures through a variety of scripting languages. 14

15 GRASS Interfaces GRASS GUI GRASS Command Line 15

16 Setting up the GRASS Workspace Run GRASS from the command prompt > grass60 Each time you run GRASS, you will be prompted for the following information: Location defines the projection, default spatial extent, and resolution for all data in the project Mapset defines collections of data within a given location. Mapsets can be used to organize data of similar types or categories Database defines where in the file system the files for the GIS will be stored for the given location and mapset. Database needs to be set to a valid directory BEFORE a location and mapset can be defined. 16

17 Setting up the GRASS Workspace (cont.) Depending upon whether your default interface is the GUI or command prompt, you will see one of the two following: GRASS Command Line GRASS GUI 17

18 Setting up the GRASS Workspace (cont.) If you want to work in an existing workspace, enter the values for the previously created location, mapset, and database. If you are working in the GUI and need to create a new workspace: select the Create New Location button, and complete the command line interface instructions for creating a new workspace. 18

19 Creating a New Location When creating a Location for the first time, you must use the command line interface. If you follow the on-screen instructions, you will be prompted to create a new Location and Mapset when you enter esc- Enter. Specifically, you will be taken through the Location creation process, then the Mapset creation process. You must have already created the specified Database location before beginning the Location creation process 19

20 Creating a New Location The general procedure for creating a new Location and associated Mapset(s) is: Determine the names to be used for the Location, Mapset, and Database. Create the Database if it does not yet exist. Run GRASS If running the GUI, select the Create New Location button. Enter the names of the new Location, Mapset, and Database in the command line interface (CLI). Hold down the ESC-key while hitting the enter/return key (<ESC><ENTER>) 20

21 Creating a New Location (cont.) If the Location does not exist, the user is prompted to create it. The following information is required to create a new location: The coordinate system for the location (i.e. UTM, LatLon, xy), and the parameters associated with that projection (if required). The default region to use for the location (i.e. bounding box N, S, E, and W boundaries). The default resolution of the location (in coordinate system units: i.e. meters for UTM, degrees for LatLon) Once the location is created, the user is returned to the original screen where the Mapset may be provided and esc-enter entered. 21

22 Creating a New Location (cont.) After esc-enter is entered, the user is prompted to confirm the creation of a new Mapset, or GRASS is started using an existing Mapset if it already exists. If using the GUI, the graphical interface is then started (in addition to the CLI), otherwise, the CLI alone is started. GRASS is now ready to be used to solve your GIS problems. 22

23 Creating a New Location - Example Given that we want to create a new location centered on the City of Albuquerque, 1x1 degrees in size, and in the UTM Zone 13 (NAD83) coordinate system, the previously described process would be performed as follows: Determine the names to be used for the Location, Mapset, and Database. Location: abq Mapset: PERMANENT Database: /home/user/grass.data 23

24 Creating a New Location - Example (cont.) Create the Database if it does not yet exist. > mkdir /home/user/grass.data Run GRASS > grass60 If running the GUI (the default in GRASS 6.0), select the Create New Location button. 24

25 Creating a New Location - Example (cont.) Enter the names of the new Location, Mapset, and Database in the command line interface (CLI), and type <ESC><ENTER>. GRASS LOCATION: This is the name of an available geographic location. -spearfishis the sample data base for which all tutorials are written. MAPSET: Every GRASS session runs under the name of a MAPSET. Associated with each MAPSET is a rectangular COORDINATE REGION and a list of any new maps created. DATABASE: This is the unix directory containing the geographic databases The REGION defaults to the entire area of the chosen LOCATION. You may change it later with the command: g.region LOCATION: MAPSET: abq (enter list for a list of locations) PERMANENT (or mapsets within a location) DATABASE: /home/user/grass.data AFTER COMPLETING ALL ANSWERS, HIT <ESC><ENTER> TO CONTINUE (OR <Ctrl-C> TO CANCEL) 25

26 Creating a New Location - Example (cont.) Prepare to create the Location by having at hand the required coordinate system, bounding box, and resolution values. Coordinate system: UTM Zone 13, NAD83 > proj -lp... utm : Universal Transverse Mercator (UTM) Cyl, Sph zone= south... This output indicates that we need to be prepared to provide one parameter, the zone, 13 in this case. 26

27 Creating a New Location - Example Bounding box coordinates: (cont.) x w x e x c y c y n y s North = y n South = y s East = x e West = x w xc, yc = , yn = ( ) ys = ( ) xe = ( ) xw = ( ) > proj +proj=utm +zone=13 +datum=nad83 <<EOF > # NE Corner > # SW Corner > EOF # NE Corner # SW Corner 27

28 Creating a New Location - Example (cont.) With the required information, the Location may now be created. When <ESC><ENTER> is entered from the GRASS Location specification screen, the Location creation process is started (if the specified Location does not already exist). LOCATION <abq> - doesn't exist Available locations: mckinley_co spearfish Would you like to create location <abq>? (y/n) [y] y 28

29 Creating a New Location - Example (cont.) The user is then reminded of the information required to create a new location. To create a new LOCATION, you will need the following information: 1. The coordinate system for the database x,y (for imagery and other unreferenced data) Latitude-Longitude UTM Other Projection 2. The zone for the UTM database and all the necessary parameters for projections other than Latitude-Longitude, x,y, and UTM 3. The coordinates of the area to become the default region and the grid resolution of this region 4. A short, one-line description or title for the location Do you have all this information? (y/n) [y] y 29

30 Creating a New Location - Example (cont.) The user is then prompted to specify the coordinate system for the new Location. Please specify the coordinate system for location <abq> A B C D x,y Latitude-Longitude UTM Other Projection RETURN to cancel > C UTM coordinate system? (y/n) [y] y 30

31 Creating a New Location - Example (cont.) The user is prompted for a one line description of the new location Please enter a one line description for location <abq> > A location centered on the City of Albuquerque ===================================================== A location centered on the City of Albuquerque ===================================================== ok? (y/n) [y] y The Geodetic Datum for the Location Do you wish to specify a geodetic datum for this location?(y/n) [y] Please specify datum name Enter 'list' for the list of available datums or 'custom' if you wish to enter custom parameters Hit RETURN to cancel request >nad83 31

32 Creating a New Location - Example (cont.) The Datum Transformation Parameters Now select Datum Transformation Parameters Enter 'list' to see the list of available Parameter sets Enter the corresponding number, or <RETURN> to cancel request >6 The Zone Enter Zone: 13 And, the Hemisphere Is this South Hemisphere (y/n) [n] n 32

33 Creating a New Location - Example (cont.) And the default region and resolution for the new Location. DEFINE THE DEFAULT REGION ====== DEFAULT REGION ======= NORTH EDGE: WEST EDGE EAST EDGE SOUTH EDGE: ============================= PROJECTION: 1 (UTM) ZONE: 13 GRID RESOLUTION East-West: 30 North-South: 30 AFTER COMPLETING ALL ANSWERS, HIT <ESC><ENTER> TO CONTINUE (OR <Ctrl-C> TO CANCEL) 33

34 Creating a New Location - Example (cont.) After completing the entry of the default region values, and entering <ESC><ENTER>, the user is asked to confirm the provided values. After confirmation, the user is taken back to the initial entry screen where a final <ESC><ENTER> will take them to the GRASS interface: GUI or CLI. 34

35 GRASS - Data Acquisition and Preprocessing

36 Basic Concepts Data acquisition and preprocessing are closely related activities in that the types of data acquired have a direct effect on the types of preprocessing required before import into GRASS. We have already looked at several technologies that both inform and support the data acquisition and preprocessing effort. Online resources such as the USGS Seamless Data Download site, RGIS, Geospatial OneStop GDAL/OGR Proj.4. The success of your data acquisition and preprocessing activities is directly measured by your ability to obtain and use data required for your specific problem. 36

37 Basic Concepts (cont.) Things that you need to consider when acquiring data for use in you projects include: The availability of metadata for the datasets (projection information, quality, contact information, appropriateness, scale, etc.) The available formats, and whether those formats match the formats that your system supports. Projection or coordinate system Coverage for your area of interest Expense, time required to acquire data Availability of non-digital data Licensing issues, ownership Each of these issues has a direct impact on the amount of effort that you will need to expend preprocessing data prior to importing it into the GIS. 37