Introduction to Geographic Information Systems in Forest Resources
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Projections and Coordinate Systems

Objectives:

• Learn how to apply different map projections to data frames.
• Examine the distortions caused by various map projections.
• Learn how to project datasets from one projection/coordinate system to another.

1. Perform drive substitution to create drives L (CD) and M (removable drive).
2. Open an existing map document
3. Check map scale and coordinates
4. Measure distances on an non-projected data frame
5. Data Frame Projection (Temporary Display)
6. Change the projection from Data Frame (Temporary Display)
7. Project a shapefile (Permanent Change)
8. Source data in decimal degrees
9. Projected source data
10. Close the project

Perform drive substitution

Perform drive substitution to create the virtual drives L and M.

Open an existing map package

2. Open the Projection_Exer map package. You should see a data frame of the world in a perspective as if you were in space.

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 1:145,534,295 (scale depends on your viewer, if you zoom in, the scale will be changed according to the viewer size). This means that 1 unit on the map is equal to 145,534,295 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 data frame, you will see the coordinates change. The coordinates are reported in map units, which in this case, are miles. Because the data frame 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 data frame is an Orthographic Projection of underlying data which are stored in decimal degrees.

Measure distances on an non-projected data frame
1. Collapse (click the "+" to the left of) the World from Space data frame and activate (Right click World Map data frame and click on Activate) the World Map data frame.
This data frame is unprojected (or Unknown), 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.

2. As you move the pointer, you will see coordinates change at the lower right of the application window (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).

3. Before measuring any distances or calculating scale, ArcGIS must be configured to report measurements in units of the user's choice. This also means that ArcGIS 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 ArcGIS 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 datasets you are using.

4. Right-click the World Map title in the table of contents and Select Properties from the popup menu. Select the Coordinate System tab. Select the coordinate system Predefined > Geographic Coordinate Systems > World WGS 1984. This sets the projection for the data frame.

5. In the General tab, set Display Units as Miles. We want our measurements to be reported in miles. OK the change.

Now you will see that the scale declaration contains a value, and when you move the pointer, the coordinates show the units are miles (at the bottom of the window).

6. Now measure some distances between selected cities. Turn on the Selected Cities layer. You will see Seattle, Guayaquil, and Manado shown in red dots, with their names on the map.

7. Maximize the size of the application window so your measurements will be more precise.

8. Click the Measure tool .

9. Select the Miles as the distance unit in the measurement tool dialog.

10. Move the pointer over Seattle, and click the mouse button. This starts a line segment. Notice the status bar in the measurement window reports the distance of the current segment as well as the total length of the line.

11. Move the pointer over Guayaquil, and double-click the mouse. You should get a distance of approximately 4,300 miles.

12. 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 data frame. 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 will use different tools covered later in the course.

It is especially important to remember that you need to set data frame properties for projection and display units before you can view map scale, or perform any measurements. You must know the units in which your data are stored!

Apply a projection to the data frame

Projections always distort map properties. We will see how a different projection distorts distance.

Reminder: Change the Projection in the data frame will only change the display temporarily but applied to every layer within this data frame.

1. Open the data frame properties again, and in the Coordinate System tab, select Predefined > Projected Coordinate Systems > World > Mercator (world) from the Type dropdown. Also make sure display units are set to Miles in the General tab.

2. Right-click the Selected Cities layer active select Zoom to layer.

3. 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.

4. Make the same measurements as you did before. This time you should get distances of about 4,800 and 14,200 miles, respectively. Because projections distort measurements, it is important to know what projection to use in order to make decisions. 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 not distorted in the X direction along the equator (since this is a secant of the projection, known as a standard parallel). But because Guayaquil and Manado are not exactly on the equator, there is a slight distortion due to Y-stretching.

You have just seen the effect that projection can have on linear measurements.

When you need to make large distance measurements on a map of the world, use unprojected data in an unprojected data frame. The measurements will be calculated on great circles, which will be more accurate than measurements made on a projected data frame, unless you have data projected specifically for minimizing distance distortion. The properties of the projection can have a profound effect on the measurements you make. When distances are calculated by different analytical processes within ArcGIS, 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.

1. Turn on the Circles.shp layer.

Notice how the circles are distorted toward the poles.

2. Turn off the Selected Cities layer.

3. Change the projection now to Hammer-Aitoff projection: Predefined > Projected Coordinate Systems > World >Hammer-Aitoff.

4. Finally, change the projection back to WGS 1984.

5. Feel free to experiment with different projections, and see how they distort the shapes and areas of features. Make some measurements of area on data frames 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 ArcGIS documentation. When areas are calculated by different analytical processes within ArcGIS, the results will be distorted according to the projection you have used.

Project a shapefile (Permanent Change)

As to no predefined projection layer, you will need to assign the projection to it by using "define projection" tools.

As to the projection change from one projection to another, use the "project" tool to make the permanent change.

You may frequently have datasets representing the same location that are already stored in different projections and coordinate systems. At ArcGIS, there is a Projection Engine that allows you to convert datasets from one projection to another. In this exercise, you will project one dataset to match another.

NOTE: the ArcGIS projection engine may be slow. When you are projecting datasets, 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.

1. Download the file projection_files.zip (Pack Forest data stored in 6 different projections) into M:\projection\.

2. Open a Windows Explorer and navigate to M:\projection\. If you are familiar with unzipping files, unzip the zip file.

3. Double-click on the zip file.

4. Click "Extract all files" at the left-hand side of the window above.

5. Click "Finish." The file stucture should be as shown below.

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 drive.

Here are the projection parameters for each dataset:

 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

6. Collapse the World Map data frame, and insert a new data frame (Insert > Data Frame), and alter its title to State Plane.

7. Add all 4 of the State Plane datasets (from the State Plane folder) from the file you downloaded and extracted. If you get an error message, click OK. This is because none of the shapefiles have projection information stored with them.

Even though these datasets 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 layers not to line up with each other.

8. Create another data frame (call this one UTM) and add both shapefiles from the UTM folder to the data frame.

If you zoom in and pan around, you will see clearly where the layers 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 dataset so it will overlay the stands correctly.

9. To understand which projection tool to use, right click the layer to see if there is any projection has been assigned into either layer. For this inspection, right-click roads_utm27 layer, select propertities, check the Source tab. You will see <Undefined> were written in the Coordinate System, which means there is no projection has been assigned to this layer yet.

10. Because neither of these datasets have projections defined, the first step will be to define the projection on each of them. Note that "defining" the projection on a dataset does not alter the dataset in any fundamental way, it just makes a record of what projection the dataset is stored in. However, in order for ArcGIS to project a dataset from one system to another, it must know what the input projection system is. This step, defining the projection, accomplishes the task of letting ArcGIS know about the dataset's current storage parameters. It is absolutely essential that you define a dataset's projection properly. If you make a mistake in defining the projection, errors will ensue any time you use the dataset.

11. Open ArcToolbox. You may want to "dock" it in your application window. After that, select Data Management Tools > Projections and Transformations and double-click the Define Projection tool.

12. Select roads_utm27 as the Input Dataset
.
13. Click the button to select a coordinate system .

14. Click Select. Select Projected Coordinate Systems > UTM > NAD 1927 > NAD 1927 UTM Zone 10N.prj. A progress window will display. When the process is complete, Close the dialog.

15. Don't skip this step! Do the same for the stands_utm83 layer, but select the projection Projected Coordinate Systems > UTM > NAD 1983 > NAD 1983 UTM Zone 10N.prj

16. After the road layer has been defined as UTM 1927, there is a fact that road layer and stand layer are not the same projection. When there is a different projection among one to the other, we need to switch road projection again to match the stand projection.
17. From ArcToolbox, now select the tool Data Management Tools > Projections and Transformations > Feature > Project.

1. Select roads_utm27 as the Input Dataset. For the Output Dataset, specify that you will be saving the dataset as roads_utm83.

2. For the Output Coordinate System, click the button to select a coordinate system . Click Import, which will allow you to copy the coordinate system from an existing data source. Navigate to the dataset for stands_utm83.

18. Click OK. A progress dialog will indicate when the process is complete. Click Close.

19. The new dataset is added as a layer to the map (if it is not added, please add it). Turn off the roads_utm27 layer and turn on the new roads_utm83 layer. You will see that it now overlaps with the stands properly.

Close the map document
1. Save the map document as projection.mxd on your removable drive.

Projections and coordinate systems may seem complex and arcane to you, but as you become more familiar with GIS and GIS datasets, you will become more capable of handling different coordinate systems and projections.

REMEMBER TO TAKE YOUR CD AND REMOVABLE DRIVE WITH YOU!!!!