PROJECTIONS AND MORE
Altering the basic geometric structure of the map can range from trivial
to difficult. Two weeks ago, we looked at the conceptual base of
projections--the context in which our maps exist. Last week we saw
applications of remote sensing. This week, we consider possible solutions
to geometric alteration of maps that lie between the trivial (just go to
View|Properties and change the projection and everything works well) to
the difficult (find a technical consultant with more software and machinery
than is routinely available in the standard computing environment).
Note: if you are loading map files from a CD, be sure to remove
the "read-only" mark from the files. Go to Windows Explorer, File|Properties
and take the check out of the box.
Concepts to look for: spatial transformation; scale; distance;
hierarchy; absolute and relative location--others?
MORE ....TO COME!
View|Properties--change the projection and all works--use the Custom settings
to change longitude on which the view is centered, and so forth.
Use the extension called "Projector."
Activate the extension; it is not part of the default extension package
installed to work directly in ArcView
To become active, an extension must be placed in the following subdirectory:
Find the extension--extensions have the file extension *.avx--look in ...\esri\Av_gis30\arcview\samples\ext
and take a copy of prjctr.avx (and .apr if you want) to the appropriate
In ArcView go to File|Extensions and click on the extension called Projector;
doing so will activate a button with an up-arrow and four dots.
Examples using SEMCOG map and City of Detroit map. Simple change
does not appear to work, although many possibilities could be tried.
It is known that the SEMCOG map is State Plane 1983,
NAD83, units=feet, and fips=2113 (South zone designation).
Projector changes only projections; it does not change the datum--the underlying
reference system against which measurements are directly referenced.
Thus, if Projector does not do the job, try something else. One other
extension, and this one is directly installed to work (by default), is
the ArcView Projection Utility--activate the extenion in File|Extensions
PRIOR TO USING THIS EXTENSION, MAKE SURE TO DISABLE ANY VIRUS CHECKING
SOFTWARE ON YOUR MACHINE; IF YOU DO NOT DO SO, THE PROGRAM IS VERY LIKELY
TO "HANG" AND GET "STUCK." Thus, if you find that you are using this
extension and it is taking forever to work, close it and go back and make
sure that your virus checking software has been disabled.
If you are not willing to disable your virus checking software, or if you
are working in an environment that will not allow you to disable it, consider
using something other than the Projection Utility.
Example converting NAD27 to NAD83, state plane coordinates. This
package does allow for datum shifts so that is one reason to consider disabling
the virus checking software. I will not do that on my laptop, however.
A script is a small program that functions, to some degree, as an extension.
ArcView comes with a number of scripts that perform various tasks.
One of them is a script that permits one to slide maps around, horizontally
or vertically. As long as displacement is the only issue, and distortion
of shape is not involved, this tool can be quite useful in moving maps
around in the View.
Activate the script called xyshift.ave
Make the View window an intermediate size and go back to the window that
lists the ArcView table of content in the left frame.
Scroll down and click on Scripts; double click Scripts.
You should now see a new blank window. Click on the Script pull-down
menu and choose "Load text file."
Navigate to the sample scripts that come with ArcView: ...\Esri\Av_gis30\ArcView\Samples\scripts
and open up the file called xyshift
You should now see a computer program in what was the blank window.
In order to "run" the script you must first "compile" it.
Click on the View window; be sure to activate the appropriate shape file.
Be sure, however, to click on the View window even if the appropriate shape
file is already active; failure to do so may produce an error message later.
Now click on the Script window
Click on the Compile button (check mark)--notice that the button just to
the right of the Compile button is not active prior to clicking on Compile.
After clicking on the Compile button, the button to the right becomes active
(it has a stick figure "running"). This newly-activated button is
the Run button: click on it.
Now the action will prompt you for various items to input. If you
need to go back to the map to figure them out, do so; the script has been
compiled and should continue to Run well.
Conceptually, this script executes a geometric tranformation, called "translation"
(literally (L.), "sliding across"). Translation is a "rigid motion"--it
does not alter shape, just slides fixed images.
Examples using City of Detroit data (freeways) and SEMCOG data (minor civil
Blue Diamond extension--builds a "world" file with which to "register"
vector maps to raster images. This extension may alter shape--it
is like building a quilt--tack the image to the map in a number of places
and what is left in between may be a bit puffy--not fit perfectly with
the map. Best fit occurs nearest registration points.
Find the extension and install it; it is called register.avx and is available
from the linked
Open up an image in ArcView--if it is not in .jpg format be sure to save
it in that format. If using *.sid files, you will have had to make
sure the MrSid extension is loaded. PhotoShop does not load Mr.Sid
files; to load them in PhotoShop, you would need to go to the web to get
an extension to enable you to do so. Thus, try saving the MrSid file
as a .jpg directly from ArcView (there will be considerable loss in resolution).
Then, set the View back to an intermediate size.
Go to Explorer and create a new folder--put the files you want to use,
shape files and related files as well as the .jpg you just created, into
this new folder.
Go back to ArcView and click on the blue diamond. Choose your .jpg
when requested to choose an image. Then, choose your map file.
Start with the .jpg and find a point in the image that you can also find
on the map--the intersection of two major roads; the intersection of a
bridge with the shore; and, so forth. Click on the little flag symbol
at the left end of the array of buttons. Choose a point on the image; then
choose the corresponding point on the map. You may wish to maximize
the window or zoom in to see better. Then back to the image, and
so forth. Execute this procedure at least six times to tack the image
to the map--the image will be stretched to "fit" the map. These points
are called "registration" or "control" points. Choosing more may
be good, but only if you can locate them with fair accuracy on both the
image and the map.
Once all the control points have been set, reduce the View window back
to intermediate size. Double click on the words under Tables--Ground
Control Points--to open up the table you have created. The last column
is labelled RMS (Root Mean Square error) and all entries will be 0.
Click on the C button to calculate the RMS. If you have more than
6 entries in the table, discard any very high values--hit the calculate
button again. Close the table. Re-open the table and look at
it and hit C again.
Now, click on the pencil symbol at the top--it will now write a "world"
file that carries the text that tells the image where to place itself in
relation to the map. Let the software decide its own name (or you
choose one--it doesn't really matter)--just keep all your files in the
Now the software will shut down the open files. Reopen the ones you
had open and you will see your freshly aligned image! If there is
too much distortion, try again!
Last option--find your technical support staff. They may reproject
files for you using ArcInfo (too expensive for the standard user).
Of critical importance--know information about your files! What projection
they are, what datum they use, what units they use and any other key factors.
Related links--if you want more scripts and extensions than the set that
comes with ArcView look on the web--build a collection--if someone offers
you one, take it. Even if it is not useful to you now, it may become
useful later on (as with maps!). Join the ArcView Users' Group--if
you are prepared to get a lot of e-mail! Join through ESRI.
Working in computing environments--complex systems in which the characteristics
of the individual element of the system do not necessarily transform, or
map, to the characteristics of the whole system. So--look for what
is good and works well within your own computing environment and supplement
it as needed from your own imagination and resource base.