Since the iPAQs arenÕt the quickest machines on the planet, it is sometimes desirable to clip out certain portions of large raster datasets for MrSID compression before loading the raster data on the iPAQs. At the moment, there does not appear to be any way to clip raster datasets using ArcGIS, so these instructions are for ArcInfo Workstation 8.2. ArcInfo Workstation is not compatible with MrSID files, so if you have a previously compressed file that you wish to clip, you will either have to accept that it isnÕt going to happen, or go back to the original dataset, clip it, and then recompress. Sorry.
p.s.
This set of instructions assumes that the image you wish to clip is
georeferenced. (If it is a .tif image, there is an accompanying .tfw
file). If this is not the case, it may be easier to crop the image first
using a graphics program such as Photoshop, then georeference the image. See georeferencing
raster data
for information on georeferencing.
ArcInfo
Workstation works only with raster data in the ESRI proprietary GRID
format. Fortunately, ArcInfo can import most types of image data into
GRID format. Go to Start > Programs > ArcGIS > ArcInfo
Workstation > Arc. A command line dialog box will appear. At the
Arc: prompt type Ôimage gridÕ (typing any command explains the usage of that
command). You will see Ð
Arc:
imagegrid
Usage:
IMAGEGRID <in_image> <out_grid | out_stack> {out_colormap_file}
{in_band}{NEAREST | BILINEAR | CUBIC} {DEFAULT | SQUARE}
Where
IMAGEGRID is the command, variables contained in <> are required, and
those contained in {} are optional. To skip certain optional variables
while filling in later ones, each unspecified variable must be held with a
Ô#Õ. For example, if I want to import the image example.tif to the GRID
examplegrd, without specifying out_color_map or in_band, but specifying
BILINEAR interpolation, I would type:
Arc:
imagegrid example.tif examplegrd # # bilinear #
Now
there is a GRID called examplegrd that is a representation of the original
example.tif image. (Note, bilinear interpolation is very time consuming.
In general, nearest (neighbor) is fine). The path taken from here to clip
images depends on whether the image is either a) grayscale or indexed, or b)
multi-band. To learn more about
these types of raster data, look here.
Grayscale
and indexed color images are the easiest types of images to crop. Once the image is imported into grid
format, display the image in Grid.
WeÕll continue to assume that the imported grayscale or indexed color
image is called examplegrd, and, for the indexed color example, that the image
has an associated color look-up table called examplegrd.lut.
Arc:
display 9999 1
Grid:
mape examplegrd
Now
we can ask Arc to draw the grid.
For a grayscale image:
For
an indexed color image:
This will draw the image in
the graphics window. Using the Pan
and Zoom tools at the upper left corner of the graphics window, you can zoom in
to the area of the image that you wish to crop. Each pan or zoom will create a new window, and each time a
new window is created, you will need to issue the appropriate draw command
again, as above. In Arc, Ô^<letter>Õ
issues the last command that begins with a specific letter, so Ô^gÕ would
reissue the entire gridpaint command without retyping it, saving some work and
effort.
When
you have zoomed to the area you wish to clip, you can enter an interactive
clipping mode:
Where
the Ô*Õ indicates that you will interactively define the clip region. Now a new cursor will appear when you
place the mouse over the graphics window.
Click the upper left and lower right corners of the clip region, and a
new grid, clippedgrd, will be created.
To export the clippedgrd
back to a tiff image, do the following:
Arc: gridimage clippedgrd # clippedgrd.tif tiff #
Although
this clipped grid will have a .tfw world file, it will not contain all
necessary projection information. Run the ProjectDefine Wizard for grids
and coverages in the ArcToolbox to update the coordinate system for this new
image. Then it is clipped and ready to be added to the ArcGIS map.
Actually,
there are three GRIDs, one for each color in the RGB TIFF file, examplegrdc1
(red), examplegrdc2 (blue) and examplegrdc3 (green). These three GRIDs
can be collectively called using a stack, which is a collection of GRIDs. The
imagegrid automatically creates a stack called examplegrd to refer to all three
pieces.
Now
we must create a clip region to clip the grids.
Arc:
create exampleclip examplegrdc1
This
creates a vector coverage called exampleclip with the same projection and tics
and examplegrdc1.
Arc:
build exampleclip poly
This
add polygons attributes to the coverage exampleclip. Now we will run a
series of commands in ArcEdit to prepare to draw the clip region. If you
are interested in the details of these commands, see the Arc manuals or ArcDoc
for help.
Arc:
display 9999 1
Arc:
ae
Arcedit:
mape exampleclip
Arcedit:
image examplegrd composite 1 2 3
Arcedit:
ec exampleclip
Arcedit:
ef poly
Arcedit:
de poly
Arcedit:
draw
Now
you should see a display window containing the GRID image that you
imported. You can use the controls at the upper left of the display
window to Pan and Zoom to the region that you would like to clip.
Arcedit:
add
A
cursor will appear when you highlight the display window. Click the 4
corners of the box you wish to clip (or draw an irregular polygon). After
clicking all of the corners, with the mouse still in the display window, hit
the Ô2Õ key on the keyboard, then the Ô9Õ key. Return to the command line
window. It should say that one polygon has been added.
Arcedit:
save
Arcedit:
q
Arc:
build exampleclip poly
Now
we will use the polygon that was just created to clip the GRID.
Arc:
grid
Grid:
gridclip examplegrdc1 clipc1 cover exampleclip
Grid:
gridclip examplegrdc2 clipc2 cover exampleclip
Grid:
gridclip examplegrdc3 clipc3 cover exampleclip
Grid
makestack clip list clipc1 clipc2 clipc3
Now
we have three new clipped grids, referred to collectively as clip. We can
export these clipped GRIDs back to a tif file.
Grid:
q
Although
this clipped grid will have a .tfw world file, it will not contain all
necessary projection information. Run the ProjectDefine Wizard for grids
and coverages in the ArcToolbox to update the coordinate system for this new
image. Then it is clipped and ready to be added to the ArcGIS map.