Geospatial data models
Start GRASS GISStart GRASS - click on GRASS icon or type
In startup pannel set GRASS GIS Database Directory to path to datasets,
for example on MS Windows,
For GRASS Location select nc_spm_08_grass7 (North Carolina, State Plane, meters) and
for GRASS Mapset create a new mapset (called e.g. HW_data_models) and
click Start GRASS session.
Change working directory:
Settings > GRASS working environment > Change working directory > select/create any directory
cd (stands for change directory) into the GUI
Console and hit Enter:
If you prefer to work in GUI, you should be able to find out yourself
the GUI equivalents for the tasks below.
Some hints for GUI are included, but from now on, most of the instructions will be provided as commands for command line.
Hint for running most of the commands in GUI - type or paste the name of the module into the command console in the Console tab and then hit Enter to open the GUI dialog.
Read the manual page for each command you are using for the first time to learn what it is doing and what the parameters mean.
Resampling to higher resolution
Resample the given raster map to higher and lower resolution (30m->10m, 30m->100m) and compare resampling by nearest neighbor with bilinear and bicubic method.
First, set the computation region extent to our study area and set resolution to 30 meters. The computational region (region for short) is set using g.region module. Here for convenience we use named region which defines both the extent and the resolution. This named region is included in the data (location) we are using but it is possible to create new named regions and use them to bookmark different study areas.
g.region region=swwake_30m -p
-p flag for g.region is used to print the region
we just set.
Then we display the 30m resolution NED elevation raster.
To resample it to 10m resolution, first set the computational region to resolution 10m,
then resample the raster using the nearest neighbor method.
Hint: To open the r.resamp.interp in GUI, type or paste the module name into the Console tab, then Enter to open the GUI dialog, don't forget to set the method to nearest under Optional tab.
g.region res=10 -p r.resamp.interp elev_ned_30m out=elev_ned10m_nn method=nearest
Display the resampled map by adding "elev_ned10m_nn" to Layer Manager
in case you don't have it in the Layer Manager already.
Alternatively, use in command line the following:
The elevation map "elev_ned10m_nn" looks the same as the original one, so now check the resampled elevation surface using the aspect map:
r.slope.aspect elevation=elev_ned10m_nn aspect=aspect_ned10m_nn
Display the resampled map by adding "aspect_ned10m_nn" to Layer Manager or in command line using:
To save the map, click in Map Display to on the button Save display to graphic file" or alternatively, use the following command:
Now, reinterpolate DEMs using bilinear and bicubic interpolation. Check the interpolated elevation surfaces using aspect maps.
r.resamp.interp elev_ned_30m out=elev_ned10m_bil meth=bilinear r.resamp.interp elev_ned_30m out=elev_ned10m_bic meth=bicubic r.slope.aspect elevation=elev_ned10m_bil aspect=aspect_ned10m_bil r.slope.aspect elevation=elev_ned10m_bic aspect=aspect_ned10m_bic d.rast aspect_ned10m_bil d.rast aspect_ned10m_bic
Save the displayed maps and in your report, compare the results with
the previously computed nearest neighbor result.
In Map Display click button Save display to graphic file, or use the following in the command line:
Why is the aspect of elevation raster map computed by the nearest neighbor method different from the one computed by bilinear interpolation?
Resampling to lower resolution
Resample to lower resolution (30m -> 100m).
First, display the original elevation and land use maps:
Then change the region resolution and resample elevation (which is a continuous field) and land use (which has discrete categories).
d.rast elev_ned_30m d.rast landuse96_28m
Explain selection of aggregation method. Can we use average also for landuse? What does mode mean?
Before the next computation, remove all map layers from the Layer Manager because we don't need to see them anymore.
g.region res=100 -p r.resamp.stats elev_ned_30m out=elev_new100m_avg method=average d.rast elev_new100m_avg
d.erase r.resamp.stats landuse96_28m out=landuse96_100m method=mode d.rast landuse96_100m
Remove or switch off the land use, elevation and aspect maps.
Converting between vector data types
Convert census blocks polygons to points using their centroids (useful for interpolating a population density trend surface):
Display census boundaries using GUI:
v.to.points census_wake2000 type=centroid out=census_centr use=vertex
Add vector "census_wake2000" Selection > Feature type > boundary (switch off the other types).
Save the displayed map in Map Display click button Save display to graphic file.
Alternatively, use the following commands to control display.
Note that in both command line and GUI you must either enter the full path to the file you are saving the image in, or you must know the current working directory.
Convert contour lines to points (useful for computing DEM from contours):
d.vect census_centr icon=basic/circle fill_color=green size=10 d.vect census_wake2000 color=red fill_color=none d.legend.vect d.out.file vect_to_points
Display the "elev_ned_contpts" points vector and zoom-in to very small area to see the actual points.
v.to.points input=elev_ned10m_cont10m output=elev_ned_contpts type=line use=vertex
d.vect elev_ned_contpts co=brown icon=basic/point size=3
Convert from vector to raster
Convert vector data to raster for use in raster-based analysis. First, adjust the computational region to resolution 200m:
Then remove all layers from the Layer Manager.
g.region swwake_30m res=200 -p
Convert vector points "schools" to raster.
As value for raster use attribute column "CORECAPACI" for core capacity.
To add legend in GUI use Add map elements > Show/hide legend and select "schools_cap_200m".
d.vect schools_wake v.info -c schools_wake v.to.rast schools_wake out=schools_cap_200m use=attr attrcol=CORECAPACI type=point d.rast schools_cap_200m d.vect streets_wake co=grey d.legend schools_cap_200m at=70,30,2,6
Now convert lines in "streets" vector to raster. Set the resolution to 30m and use speed limit attribute.
g.region res=30 -p v.to.rast streets_wake out=streets_speed_30m use=attr attrcol=SPEED type=line
If you haven't done this already, add remove all other map layers
from Layer Manager and add "streets_speed_30m" raster layer.
Add legend for "streets_speed_30m" raster using GUI in Map Display:
Add legend > Set Options > Advanced > List of discrete cat numbers and type in speed limits 25,35,45,55,65; move legend with mouse as needed.
Alternatively, use the following commands:
d.erase d.rast streets_speed_30m d.legend streets_speed_30m at=5,30,2,5 use=25,35,45,55,65
Save the displayed map. In Map Display click button Save display to graphic file, or use the following.
Convert from raster to vectorConvert raster lines to vector lines.
First, set the region and remove map layers from Layer Manager. Then do the conversion.
Explain why we are using r.thin module. You may want to remove all previously used layers from the Layer Manager before you start these new computations.
d.erase g.region raster=streams_derived -p d.rast streams_derived r.thin streams_derived output=streams_derived_t r.to.vect streams_derived_t output=streams_derived_t type=line
Visually compare the result with streams digitized from airphotos.
d.vect streams_derived_t color=blue d.vect streams color=red
Save the displayed map (in Map Display click button Save display to graphic file).
Convert raster areas representing basins to vector polygons.
Use raster value as category number (flag -v) and
display vector polygons filled with random colors.
In GUI: Add vector > Colors > Switch on Random colors. You may want to remove all previously used layers from the Layer Manager before you start these new computations.
g.region raster=basin_50K -p d.erase d.rast basin_50K r.to.vect -sv basin_50K output=basin_50Kval type=area d.vect -c basin_50Kval d.vect streams color=blue
Save the displayed map either using GUI or using the following in case you are working in the command line.