Introduction to
Geographic Information Systems in Forest Resources |
Exercise: Setting View Properties (Projections and Coordinate
Systems)
Objectives:
- Learn how to apply different map projections to views.
- Examine the distortions caused by various map projections.
- Learn how to project data sets from one projection/coordinate system to
another.
Open an existing project
Check map scale and coordinates
Measure distances on an unprojected view
Apply a projection to the view
Change the projection
Project a shapefile
Source data in decimal degrees
Projected source data
Close the project
Open an existing project
- If you have completed previous exercises, delete everything from your lab
exercise removable disk.
- Download the project file projection.apr
to your removable disk.
- Use the CFR 250 Startup Project to open the project file you just
downloaded.
- Open the project. You should see a view of the world in a perspective as
if you were in space, directly over Seattle.
Check map scale and coordinates
The current map scale will be displayed at the right hand side of the tool
bar. Due to the monitor and resolution this example was developed on, the displayed
scale is 1:162,753,135. This means that 1 unit on the map is equal to 162,753,135
units measured on the earth's surface. Your scale representative fraction may
differ from this, depending on your monitor's resolution.
To the right of the scale declaration is a display of the coordinate where
the pointer is located. As you move the pointer around the map view, you will
see the coordinates change. The coordinates are reported in map units,
which in this case, are miles. Because the view is centered on Seattle, the
coordinate values tell you how far away you are from Seattle (negative values
will show that the pointer is located to the south and/or west of Seattle).
This view is an Orthographic
Projection of underlying data which are stored in decimal degrees.
Measure distances on an unprojected view
- Close the World from Space view and open the World Map view.
This view is unprojected, meaning that the data are stored as geographic
coordinates (also know as latitude-longitude, or simply, lat-long).
These coordinates are measurements of angles on a sphere, rather than Cartesian
coordinates measured on a plane. Although the display is planar (displayed
similar in appearance to the Plate Carée projection), the underlying
data are stored and accessed as decimal degrees.
- As you move the pointer, you will see coordinates (reported in decimal degrees,
with the Greenwich Meridian is the origin of the X axis and the Equator is
the origin of the Y axis).
- You may notice that the scale declaration is null. Before measuring any
distances or calculating scale, ArcView 3.x must be configured to report measurements
in units of the user's choice. This also means that ArcView 3.x must be told
what units the data are stored in. The reason for needing to set both map
(storage) and distance (display) units is that ArcView 3.x must calculate
a storage-unit to display-unit transformation. Without knowing what the storage
units are, it is impossible to calculate scale or to perform any measurements.
For this reason alone, you should always know the projection and
coordinate system of any data sets you are using.
- Select View Properties from the menu.
- In the Map Units dropdown, select decimal degrees. The data
we are using in this view are stored in decimal degrees of latitude and longitude.
- From the Distance Units dropdown, select miles. We want our
measurements to be reported in miles. OK the change.
Now you will see that the scale declaration at the right hand side of the
tool bar contains a value.
- Now measure some distances between selected cities. Turn on the Selected
Cities theme. You will see Seattle, Guayaquil, Punta Arenas, and Manado
selected, with their names on the map.
- Click the Measure tool
.
- Move the pointer over Seattle, and click the mouse button. This starts a
line segment. Notice the status bar in the lower right of the ArcView 3.x
application window reports the distance of the current segment as well as
the total length of the line.
- Move the pointer over Punta Arenas, and double-click the mouse. You should
get a distance of approximately 7,600 miles.
- Now measure the distance between Guayaquil and Manado. This distance should
measure approximately 10,700 miles.
You have just made some simple measurements on a view. Because of the scale
at which we are viewing the data, these measurements are approximate, and will
differ if you repeat the measurements. For more precise measurements we use
different tools covered later in the course.
It is especially important to remember that you need to set view properties
for Map Units before you can view map scale, and you must also set the Distance
units before you can perform any measurements. You must know the units
in which your data are stored!
Apply a projection to the view
Projections always distort map properties. We will see how a different projection
projection distorts distance.
- From the View > Properties menu, click on the Projection
button, and select Mercator from the Type dropdown.
- Make the Selected Cities theme active and zoom to its extent
.
- Notice how different the map looks. The area near the equator is stretched
out to the north, and the northern latitudes are compressed. The Mercator
projection was developed to give correct compass bearings between locations
for seafaring.
- Make the same measurements as you did before. This time you should get distances
of about 8,900 and 14,100 miles, respectively. Because projections distort
measurements, it is important to know what projection to use in order to make
decisions. If you were planning a flight from Seattle to Punta Arenas, it
would be very important to choose a projection that minimizes distance distortion,
or you may run out of gas somewhere over the Pacific Ocean!
In the Mercator projection, in the Y direction, distances are stretched increasingly
near the poles. This is the reason we get distorted Y measurements. The projection
is also distorted in the X direction for an unknown reason (ArcView's help
says "The only true scale is along the Equator," which should imply
that the distance between two cities very close to the equator should be accurate
in the Mercator projection, but this is obviously not the case).
You have just seen the effect that projection can have on linear measurements.
When you need to make large measurements on a map of the world, use unprojected
data in an unprojected view. The measurements will be calculated on great circles,
which will be more accurate than measurements made on a projected view. The
properties of the projection can have a profound effect on the measurements
you make. When distances are calculated by different analytical processes within
ArcView 3.x, the results will be distorted according to the projection you have
used.
Change the projection
You have just seen how a difference in projection can change distance measurements.
Now see how projections can also distort shape and area.
- Turn off the Selected Cities theme.
- Turn on the Circles.shp theme.
Notice how the circles are distorted.
- Change the projection now to Hammer-Aitoff.
- Finally, change the projection back to <None>.
- Feel free to experiment with different projections, and see how they distort
the shapes and areas of features. Make some measurements of area on views
with different projections using any of the polygon drawing tools to see the
effect of projections on area measurements.
You have just seen the effect that projection can have on shape and area. Again,
if you need accurate area measurements, make sure to choose a projection that
correctly represents area. See the reference
pages on projections for details on the properties of different projections,
or look in ArcView 3.x documentation. When areas are calculated by different
analytical processes within ArcView 3.x, the results will be distorted according
to the projection you have used.
Project a shapefile
There are two major methods for projecting shapefiles. The first method is
used when you have data stored in decimal degrees (known also as "lat/long,"
"geographic," or "unprojected"), and you want to save a
copy of the data set in a different projection.
Method 1 (source data are stored unprojected, i.e.,
in decimal degrees of latitude and longitude):
- Project the World Map view to the custom projection (make sure to
click the Custom radio button and alter all of the parameters on the
dialog):
This "custom" projection is equivalent to the Washington State Plane
South Zone NAD83.
- The view will now appear to be in this projection. Your view may look a
little different than this if you are zoomed to a different scale or location.
- Make the Countries ('98) theme active.
Make sure this is the active theme. If you have a different theme active,
it will be converted/projected. From the menu, select Theme >
Convert to shapefile. Make sure you activate the theme you want to save!!
- You will need to select a location and name for the new shapefile. Call
it cntry98lambert.shp on your removable disk.
- You will be asked if you want to save the shapefile in projected units.
Click Yes. This will create a new shapefile that is no longer stored
in latitude and longitude spherical coordinates, but in projected (State Plane)
feet units.
- Because you have saved the new shapefile in projected units, it will not
be added to the view. ArcView 3.x will attempt to prevent you from putting
projected and unprojected themes in the same view.
- Open a new view and add the new data set to the view. Move the pointer around
to see the coordinate values. Look at View > Properties at the map
and distance units (they will both be "unknown"). This tells you
the new data set is now stored in the projection of the previous view.
If your data do not look like this, you may have overlooked an
important step (making the correct theme active).
You have just created a new shapefile stored in the projection you specified
in the view. The source shapefile was stored unprojected, in units of decimal
degrees of longitude and latitude. The new shapefile is stored in the Lambert
projection with units of feet. The difference between altering a view's projection
properties and creating a projected shapefile is that in the former case, the
data have not been altered, whereas in the latter case, the actual data on the
disk have been altered, and you are left with a new data set stored
in the new projection.
Method 2 (source data are already projected):
You may frequently have data sets representing the same location that are already
stored in different projections and coordinate systems. At ArcView 3.x, there
is a Projection Engine that allows you to convert data sets from one projection
to another. In this exercise, you will project one data set to match another.
NOTE: the ArcView 3.x projection engine is slow. When you are projecting data
sets, you will need to be patient, especially if you are on a slow computer.
A way to increase the speed is to turn off any anti-virus software during the
use of the extension.
- Download the file projection_files.exe
(a self-extracting zip file of Pack Forest data stored in 6 different projections)
to your removable drive.
- Open a Windows Explorer and double-click on each of these files. The WinZip
self-extractor will open. When asked where you want to extract the files,
enter the root directory of your removable drive, and click the Unzip
button.
The default Unzip to folder location will be the current system temp
directory. Do not unzip to this directory, but to your removable disk.
The WinZip Self-Extractor will indicate that 36 files were unzipped. These
files include both the data sources and the legend files for the 6 shapefiles.
This will extract the components of the 6 shapefiles needed for the exercise,
roads_utm27, roads_sps83, stands_utm83, and stands_sps27.
The files will be placed in separate directories (utm and state_plane)
on your removable disk.
Here are the projection parameters for each data set:
| name |
data |
projection |
zone |
datum |
units |
| soils_spn27 |
forest soils |
State Plane |
WA-North |
NAD27 |
feet |
| nwi_spn83 |
National Wetland Inventory |
State Plane |
WA-North |
NAD83 |
feet |
| streams_sps83 |
streams |
State Plane |
WA-South |
NAD83 |
feet |
| pls_sec_sps27 |
PLSS sections |
State Plane |
WA-South |
NAD27 |
feet |
| roads_utm27 |
roads |
UTM |
10 |
NAD27 |
meters |
| soils_utm83 |
soils |
UTM |
10 |
NAD83 |
meters |
- Create a new view, and alter its title to VState Plane. Use View
> Properties and replace the current name of the view.
- Add all 4 of the State Plane data sets from the file you downloaded and
extracted. Zoom out to full extent.
Even though these data sets represent features at the same real-world location,
the projections they are stored in differ, which is why they do not overlap
as expected. Although all 4 of these are stored in the State Plane system,
they differ in both the State Plane zone and the datum in which they are stored
. These differences are what cause the themes not to line up with each other
- Create another view (call this one VUTM) and add both *utm*
shapefiles to the view.
If you zoom in and pan around, you will see clearly where the themes do not
line up where they should.
The difference between NAD27 and NAD83 is less profound when using the UTM
system than when using State Plane for Washington State, but you should still
be able to see how the features do not line up as expected.
We will project the roads data set so it will overlay the stands correctly..
- In your view with the UTM themes, make the Roads_utm27.shp theme
active. This theme will be projected from UTM Zone 10N NAD27 to UTM Zone 10N
NAD83.
- From the Start Button, select the ArcView 3.x Projection Utility
(in the same place where you start ArcView). The utility may take several
seconds to open. If the utility does not appear to open immediately, DO
NOT open multiple instances of the projection utility or you will slow
your computer to a virtual halt. Just be patient and the utility will open.
IMPORTANT NOTE: the Projection wizard is not an integral part of ArcView 3.x,
so it will not show up in the application window. In addition, to make matters
worse, it may not show up in the task bar. Make sure to use <ALT-TAB>
to switch among active applications.
- After a brief wait, the ArcView 3.x Projection Utility Wizard.
- Click Browse and select the correct file.
Note that the Coordinate System is shown to be unknown. There are no
data within the data source to indicate its projection; the only reason you
know the projection of this data set is because you have been told. Whenever
you get data from any source, you should always find out or ask what the projection
and coordinate system parameters are.
- Click the Next button to enter input parameters. You may need to
wait a minute for the wizard to change.
- Check the Show Advanced Options checkbox.
- Click the Projected radio button for Coordinate System Type.
- In the Name dropdown, select NAD_1927_UTM_Zone_10N [26710]. This
is the current projection of the roads data.
- Units will automatically be set to Meter [9001].
- Click the Datum tab.
Because this specific projection transformation includes a datum shift,
select the Geographic Transformation NAD_1927_To_NAD_1983_NADCON.
This will handle the difference in datums between NAD27 and NAD83 for
data sets within the continental USA.
- Click the Next button. Because the
input parameters were unknown, ArcView 3.x gives you the option to save the
input parameters with the shapefile
Click No. This would create a file (roads_utm27.prj)
containing the projection description for this data set.
Saving projection information with a shapefile will simplify future projections
on the shapefile, but if you have made a mistake in any of the input parameters,
you will need to delete the prj file before you can make any other
projections of the shapefile. You should click Yes only if you are
certain that the input parameters you have specified correctly match those
of the shapefile.
- Now select the output system.
- Coordinate System Type should be set to Projected.
- Select NAD_1983_UTM_Zone_10N [26910] as the projection name.
- The units will be automatically set to Meter [9001].
- Click Next to to to the step for defining the output data set name.
- The default name of the new shapefile is Newshape.shp. This name
is not very descriptive, so click the Browse button to define a new
name.
- Set the new name to roads_utm83.shp within the utm directory.
This file name will tell you more about the data than the default file name.
Click the Save button.
- Now the name is set correctly.
Click Next to move on.
- Review your setup to make sure your input and output parameters are correct.
They should match these:
If you have made any errors, you can go Back and fix any of them. If
you are satisfied, click the Finish button. It will take a minute or
more to complete.
- A progress indicator will display momentarily. After the progress indicator
terminates, ArcView 3.x will inform you that the process is complete.
- A dialog may ask if you want to add the new shapefile to the project.
If this dialog does not appear, add the theme manually.
- Select the new shapefile.
- Add the theme to the view containing your other UTM data. (Note: this is
why the views were renamed earlier.)
- With the theme added, you will see that the roads are now in the correct
location.
You have just projected a theme from one projection to another. Use this technique
whenever you have vector data sets that have incompatible projections. In order
to be able to project data sets, you will need to know both the input and output
projections. The other limitation is that the ArcView 3.x Projection engine
only handles vector data; if you have grid or image data, you cannot use ArcView
3.x to project those data sets. Without using an extra
3rd party extension, it is only possible to project vector data to match
the projection of a grid, and not vice-versa.
Close the project
- Make the Project window active by selecting projection.apr from the
Windows menu.
- From the File menu, select Close Project.
- Save the project as projection.apr on your removable disk.
- Remember to use the startup project to reopen any project built with the
startup project.
Projections and coordinate systems may seem complex and arcane to you, but
as you become more familiar with GIS and GIS data sets, you will become more
capable of handling different coordinate systems and projections.
Syllabus