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To learn about basic raster commands you will need to load a digital
elevation model of Shenandoah National Park.
To import the GLOBE binary digital elevation model to into a GRASS raster
file:
- Type r.in.bin
input=shenandoah_dem.bin output=elevation bytes=4
- input=name:
the name of the GLOBE binary digital elevation model
- output=name:
the name of the new raster map
- bytes=value:
number of bytes per cell
To load the raster image into the monitor:
- Type d.rast
map=elevation
- map=name:
the name of the raster map to open
The digital elevation model should now be visible in the monitor and
will look similar to Figure 3.1 below.
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Figure 3.1: Digital elevation
model in a monitor
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Displaying the scale
The scale for a raster map can be plotted over the map. To draw the scale:
- Type d.scale
bcolor=black
tcolor=white at=0,0
- bcolor=name:
background color underlying the scale and north arrow
- options: white,
red,
orange,
yellow,
green,
blue,
indigo,
violet,
magenta,
brown,
gray, and black
- tcolor=name:
foreground color in which the text, scale, and north arrow are drawn
- options: white,
red,
orange,
yellow,
green,
blue,
indigo,
violet,
magenta,
brown,
gray, and black
- at=x,y
:geographic coordinates designating the location at which the upper
left corner of the scale is to be placed
The raster map will now have a black and white scale on it in the monitor,
and will look similar to Figure 3.2 below.
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Figure 3.2: Digital elevation
model raster map with scale
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Displaying the legend
The legend for a raster map can be added to the monitor. Use d.erase
to remove the old map and d.rast
to reload the digital elevation map. To draw the legend:
- Type d.legend
map=elevation color=white lines=100
- map=name
: the name of the raster map
- color=name:
the color of the legend text
- options: white,
red,
orange,
yellow,
green,
blue,
indigo,
violet,
magenta,
brown,
gray, and black
- lines=value:
number of lines to appear in the map legend
IMPORTANT: This elevation raster map has
over 65,000 legend entries. Because not all 65,000 can be viewed in the
monitor, the lines value was set to 100 in order to see the first 100.
Raster maps with less entries will be able to display the entire legend
without specifying the lines value.
The raster map will now have a legend on it in the monitor, and will
look similar to Figure 3.3 below.
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Figure 3.3: Digital elevation
model raster map with partial legend
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Finding the coordinates of a point
The coordinates of a particular point in a raster map can be printed
to the command line interface. To print the location of a point:
- Type d.where
-1
- -1:
print the location of only the first point clicked on the map
The command line interface will prompt you to use the mouse to click
on the location of the point you want to locate. This prompt will look
similar to Figure 3.4 below.
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Figure 3.4: Where point prompt
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- Place the mouse pointer over the location on the map you want to identify,
and click the left mouse button.
The location of the point will be printed to the command line interface.
This location will look similar to Figure 3.5 below (your screen may look
different depending on what location you clicked on).
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Figure 3.5: Where point information
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Overlaying cell category values
The numerical cell category values can be printed in each area of a raster
map. A grid is drawn to outline each area. The numerical values are printed
in black or white depending on the underlying raster color. Due to the
large size of the map, in order to see the numerical values you must zoom
in (using d.zoom)
on a portion of the elevation map before running this command. Use d.erase
to remove the old map and d.rast
to reload the zoomed-in digital elevation map. To overlay the category
values and draw the grid in gray:
- Type d.rast.num
map=elevation
grid_color=gray
- map=name
: the name of the raster map
- grid_color=name:
the color of the legend text
- options: white,
red,
orange,
yellow,
green,
blue,
indigo,
violet,
magenta,
brown,
gray, and black
The zoomed-in raster map will now have an overlay of grid areas and cell
values on it, and will look similar to Figure 3.6 below (your screen may
look different depending on which part of the map you zoomed in on).
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Figure 3.6: Digital elevation
model raster map with grid areas and cell values
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Creating neighbor-manipulated map
A new raster map can be created in which each cell category value is
assigned a value based on a calculation applied to the surrounding cells.
To create a new map with the average of the category values assigned to
the 8 cells around each cell:
- Type r.neighbors
input=elevation output=dem_neighbors
method=average size=3
- input=name:
the name of the original raster map
- output=name:
the name of the new raster map with the new cell categories
- method=name:
the operation performed on the neighboring cells
- options: average,
median,
mode,
minimum,
maximum,
stddev,
variance,
diversity, and interspersion
- size=value:
the neighborhood size specifies which cells surrounding any given
cell fall into the neighborhood for that cell. The size must be
an odd integer.
To view the neighbor-manipulated map use d.erase
to remove the old map and d.rast
to open dem_neighbors.
The neighbor-manipulated map should now be visible in the monitor and
will look similar to Figure 3.7 below. You can open the original file,
elevation,
to compare the two.
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Figure 3.7: Neighbor-manipulated
digital elevation model raster map
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Creating and removing masks
A mask can be placed on a raster map. This mask will block out certain
areas of a map from analysis. While a mask is active, GRASS commands will
only act on data falling inside the masked area. To create a new mask
on the map:
The command line will print a menu that looks like Figure 3.8 below.
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Figure 3.8: Mask menu
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- Type 2
to create a new mask on the raster map.
The command line interface will prompt you to name an existing raster
map layer for masking. This prompt will look similar to Figure 3.9 below.
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Figure 3.9: Prompt for mask
creation
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- Type in the name of the raster file.
A listing of the map's categories will be shown, that looks like Figure
3.10 below.
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Figure 3.10: Raster map category
list for mask creation
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The command line interface will prompt you to assign a value of
1 or
0 to each map category. Areas assigned
category value 1
will become part of the mask's surface, while areas assigned category
value 0
will become "no data" areas in the MASK file. Follow
the directions at the bottom of the screen until at entire mask has been
set.
To remove the current mask:
- Type r.mask
- After the menu shown in Figure 3.9 appears, type
1 to remove the current mask on
the raster map.
Creating or modifying the color table
The color table of the raster image can be created (if it does not exist)
or modified. To modify color table to a gray scale for the digital elevation
model:
- Type r.colors
map=elevation color=grey
- map=name
: the name of the raster map
- color=type:
the color table to use
- options: aspect
(aspect oriented grey colors), grey
(grey scale), grey.eq
(histogram-equalized grey scale), gyr
(green through yellow to red colors), rainbow
(rainbow color table), ramp
(color ramp), random
(random color table), ryg
(red through yellow to green colors), wave
(color wave)
The digital elevation model should now be visible in a gray scale in
the monitor and will look similar to Figure 3.12 below.
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Figure 3.12: A gray scale digital
elevation model
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Use r.colors
to reset the color table to a rainbow color table.
Drawing vector features
Vector features can be drawn on top of the raster map. Use d.erase
to remove the old map and d.rast
to reload the digital elevation map. To draw an area on the digital elevation
model:
The command line will print a menu that looks similar to Figure 3.13
below.
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Figure 3.13: Menu for drawing
vector features
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- Type "A"
to define a vector area.
The command line will prompt you to define an area. The prompt will look
similar to Figure 3.14 below.
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Figure 3.14: Prompt for drawing
an area vector feature
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- Click the left button to see a print out of the location of your mouse.
These locations will be printed to the command line interface and will
look similar to Figure 3.15 below.
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Figure 3.15: Mouse locations
defined during vector feature creation
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- Click the middle mouse button to define a point on the area.
- Click the right mouse button to stop defining points and close the
area.
The command line will prompt you to enter a category number for the area
and a label for the category number. These prompts will look similar to
Figure 3.16 below.
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Figure 3.16: Define area vector
feature characteristics
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- Type the new category number.
- Type the new label for the category number.
The command line will print your entries and prompt you to double check
your definition. This prompt will look similar to Figure 3.17 below.
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Figure 3.17: Check area vector
feature characteristics
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- Type "y"
to accept the new area, type "n"
to discard the definition (typing "n"
will return the command prompt to Figure 3.16).
The command line will print the menu that looks similar to Figure 3.13
above. The procedures for creating lines or circles are similar to the
above procedure for creating an area. Once you have defined all the vector
features you would like:
- Type "Q"
to quit and define a new raster map.
The command line will prompt you to enter the name of the new raster
map. This prompt will be similar to Figure 3.18 below.
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Figure 3.18: Prompt to enter
name for new raster file
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- Type the name of the new raster map.
You can use d.erase
to remove the old map and d.rast
to open the new raster map with the vector features.
Displaying category information
GRASS can print the category information for a raster map to the command
line interface. Use d.erase
to remove the old map and d.rast
to reload the digital elevation map. To print category values (for categories
1, 2, and 5 through 10) and labels for the digital elevation model:
- Type r.cats
map=elevation cats=1,2,5-10
- map=name
: the name of the raster map
- cats=value
or range:
the category values to display
A category list will be printed to the command line interface and will
look similar to Figure 3.19 below.
IMPORTANT: If this map had labels for each
category, they would be listed to the right of the category number.
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Figure 3.19: Category list
for the digital elevation model
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Generating statistics report
Statistics reports about a raster map can be generated and printed to
the command line interface. To create a statistics reports for digital
elevation map that reports the number of each category, the area covered
in square meters and the percent coverage of each category:
- Type r.report
map=elevation units=c,me,p
- map=name
: the name of the raster map
- units=name(s):
the unit(s) to report in
- options: mi(les),
f(eet),
me(ters),
k(ilometers),
a(cres),
h(ectacres),
c(ounts),
p(ercent cover)
A statistics report will be printed to the command line interface. Due
to the large number of categories, the entire report can not be reference
as a Figure. Instead your report should have a beginning that looks similar
to Figure 3.20.1 below, and an end that looks similar to Figure 3.20.2
below.
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Figure 3.20.1: Beginning of
statistics report for the digital elevation model
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Figure 3.20.2: End of statistics
report for the digital elevation model
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Converting a raster map to a vector map
A raster map can be converted into another vector map. To convert the
digital elevation map to a vector map of defined lines:
- Type r.line
input=elevation output=dem_vector
type=line
- input=name:
the name of the original raster map
- output=name:
the name of the new vector map
- type=name:
type extracted vectors a linear edges or area edges
To convert the digital elevation map to a vector map of area edge features:
- Type r.poly
-l input=elevation output=dem_vector2
- -l:
the new map will have smoothed corners
- input=name:
the name of the original raster map
- output=name:
the name of the new vector map
IMPORTANT: The created vector maps can be
viewed using the vector display command which is explained later in the
tutorial.
Changing the Mapset
Before continuing you will need to change the mapset to the next area
(Rockingham County):
- Type g.mapsets
mapset=rockingham
- mapset=name:
name of existing GRASS mapset under the current location
Click the Next
button to learn basic vector manipulations.
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