At larger map scales, the datum that is the context in which geographic data are measured becomes very important. Here we will explore how the software helps us work with datums, and in the process, hopefully gain a better understanding of the nature and importance of datums.
You run across the word transformation in many contexts in the GIS world. In Geography 484 you saw it used in the context of georeferencing. It referred to the mathematics that enable the conversion of data in one 2D Cartesian coordinate system to another. The affine transformation was used in that exercise. As you will see in this portion of the lesson, the term transformation is also used to refer to the more complicated mathematics involved in converting from one datum to another.
Another technical detail that will come into play in the following exercise has to do with how degree values are expressed. The GIS needs to do its calculations in terms of decimal numbers, so it uses decimal degrees whenever spherical coordinates are involved. But, possibly for reasons having to do with user-friendliness when you are asked to supply coordinate values in terms of degrees, it is often in terms of Degrees Minutes Seconds, or DMS. Though, this is not always the case, as you saw in the previous Parts of the lesson when you modified projection parameters.
A. A dataset without a defined coordinate system: A GIS no-no
- In ArcCatalog, navigate to your Lesson7 folder.
- Expand the PR folder. You should see a coverage named puerto_rico.
- What is its defined coordinate system? _______________
See about defining a projection in the Concept Gallery.
B. Define the coordinate system of a coverage
Your trusted colleague--s/he had better be--tells you that the coordinate system of the puerto_rico coverage is defined by the following:
- projection: Albers Equal Area Conic
- units: meters
- false easting and false northing: 0
- longitude of the center: -66 degrees 15 minutes 0 seconds (-66.25 degrees)
- first standard parallel: 17 degrees 52 minutes 30 seconds (17.875 degrees)
- second standard parallel: 18 degrees 30 minutes 0 seconds (18.5 degrees)
- latitude of origin: 18 degrees 0 minutes 0 seconds (18.0 degrees)
- x and y shift: 0
- datum: NAD27
- Open ArcToolbox.
- Expand the Data Management Tools toolbox.
- Expand the Projections and Transformations tool set.
- Double-click the Define Projection tool.
- In the Input Dataset or Feature Class slot, navigate to the puerto_rico coverage.
- At the right end of the Coordinate System slot, click the icon. This will open the Spatial Reference Properties window.
- Click the Select button | Projected Coordinate Systems | Continental | North America | choose North America Albers Equal Area Conic.prj
Before selecting OK, right-click and select Copy and Modify... (Or if using ArcGIS version 10.1 or previous, click the Add button to get back at the Spatial Reference Properties window, and then click the Modify... button).
This will bring up the Projected Coordinate System Properties window.
- In the Projection area of the window supply the decimal versions of the parameters listed above.
- In the Geographic Coordinate System area of the window, click the Select... button | North America | choose NAD 1927.prj | click the Add button.
- Click OK to close the Projected Coordinate System Properties window.
- Review the Details list of parameters for accuracy, then click OK to close the Spatial Reference Properties window.
- Click the OK button in the Define Projection window.
Let's give the puerto_rico coverage a more descriptive name. Rename the puerto_rico coverage pr_nad27.
C. Create a new personal geodatabase
We are going to re-use the puerto_rico name, because it is good general umbrella for the work we are about to perform.
- In the PR folder, create a personal geodatabase called puerto_rico.
D. Convert a coverage to a feature class
In ArcToolbox, I want you to convert the pr_nad27 coverage to a feature class. Give the feature class the same name: pr_nad27.
Expand the Conversion Tools toolbox.
Expand the To Geodatabase tool set.
Use the Feature Class To Feature Class tool to convert the pr_nad27 coverage to a feature class. Be certain to use the polygons from the coverage, not the tics or the arcs.
Put the new feature class in your newly created puerto_rico.mdb geodatabase.
Name the new feature class pr_nad27.
E. Make a second version of the Puerto Rico coastline layer, but based on the NAD83 datum
Read through this section before you charge into it. I am going to expect you to be able to interact with the Project tool without providing you with step-by-step instructions.
- Use the Project tool to project the pr_nad27 feature class to a feature class in terms of NAD83 called pr_nad83.
Put the new feature class in your puerto_rico.mdb geodatabase.
When you get to the Spatial Reference Properties window you have the choice of Selecting, Importing, or defining a New projection. We are not changing the type of map projection we are using in this case; we are going to create a new dataset in the same map projection, but based on a different datum (which will change the projected coordinate data). So, we are not going to change any of the projection parameters of the conic projection. Therefore, to save us from having to enter again the central meridian, the standard parallels, etc., do the following when you get to the Spatial Reference Properties window:
- In the Spatial Reference Properties window click the Add Coordinate System button (the graticule icon) and select Import... (or in ArcGIS version 10.1 or earlier click the Import... button).
- Navigate to the pr_nad27 feature class Add button.
- Now, back in the Spatial Reference Properties window right-click the highlighted NAD_1927_Albers coordinate system and select Copy and Modify... (or in ArcGIS version 10.1 or earlier click the Modify... button). The projection parameters are what we want them to be.
What we need to do here is select a different Geographic Coordinate System:
- Hit the Change... button (or Select... button for 10.1 or earlier) for the Geographic Coordinate System.
- Proceed, via Geographic Coordinate Systems | North America to select the NAD 1983 datum designation. Click the OK button.
- Back in the Projected Coordinate System Properties window, click the Apply button. Notice that text in the Name: slot becomes highlighted.
- Change the Name: to NAD_1983_Albers.
- Click the OK button, twice, to get back to the the Project tool dialog box.
If you click the OK button now you will get the message: Undefined Geographic Transformation
We are transforming from the NAD27 datum to the NAD83 datum, and we need to specify as much.
- Expand the Geographic Transformation (optional) dropdown window.
This is a list of possible methods that will allow us to transform from the NAD27 to the NAD83 datum. We need to make a choice here, but we need some help choosing
Being careful not to end your Project tool session:
- Find the PEgt_v9.pdf file in your Lesson7 data folder, and open it.
The contents of this document associate the sometimes-cryptic entries you see in the using: list to the part of the Earth's surface that the given datum transformation is appropriate for. [This PDF document pertains to the Projection Engine of Version 9 of the GIS software (hence the file name).]
Go to page 11 of the PDF file. That is where the NAD27-to-other methods are listed.
A third of the way down the page, in the right-hand column, find Puerto Rico.
Take note of the associated NAD_1927_To_NAD_1983_PR_VI method name in the left-hand column.
- Go back to your Project tool session, and in the Geographic Transformation (optional) dropdown window select the NAD_1927_To_NAD_1983_PR_VI transformation entry.
Then click the OK button.
- Close the Project window when the process completes.
It may take a few seconds to create the new dataset; there are over 200 polygons in our Puerto Rico dataset.
For some more discussion regarding Transformations, Projecting-on-the-Fly and Reprojecting look in the Concept Gallery.
F. Let's map the two Puerto Rico layers
You will now see that your ability to deal with datum transformation in the context of creating a new dataset extends to viewing multiple datasets that are cast in different datums.
- Open a new map document in ArcMap, and save it as puerto_rico to your PR folder.
- With the Add Data tool, add only the pr_nad83 feature class.
- With the Add Data tool again, add the pr_nad27 feature class. (Do NOT drag-and-drop from ArcCatalog).
You will get a Warning window, explaining that the geographic coordinate systems (the datums) differ. You knew that, right? (Geographic Coordinate Systems Warning box)
Because the datasets are cast in different datums, you are being given the option here to inform the software on how to transform the NAD27-based data to the datum of the coordinate system of the data frame, for the sake of the display. Recall that the coordinate system of the dataset that is first added to an ArcMap session defines the coordinate system of the data frame/map display area.
For the sake of this lesson let's pretend we are in a hurray and ignore the warning (but you never do that, do you?).
- Click Close in the warning window without making a proper transformation.
- Zoom into the southwestern portion of the largest island's coastline. You should see that the coastlines of the two data layers are offset from each other because they have different geographic coordinate systems (datums), and no transformation is being used (for the sake of your viewing needs). If you need to change the colors of the outlines and/or fills so it is visually clear the two datasets are present and offset from each other, do so. Do you understand from this demonstration why errors between geographic coordinate systems may have larger effects when working with larger map scales? Compare this to how the different projections and their distortions affected area in Part III.
Make a screen capture showing the misaligned coastlines. This screen capture is part of your deliverables.
G. Datum transformation redux
So let's now handle this situation as we should have the first time around.
- Bring up the Data Frame Properties window.
- In the Coordinate System tab, click the Transformations...button. Expand the using: list. Do you see a familiar entry there?
- Select the NAD_1927_To_NAD_1983_PR_VI transformation entry from the list, and click OK.
- Click OK in the Data Frame Properties window.
The polygons on the pr_nad27 layer should now align with those on the pr_nad83 layer.
- Depicting the same area and extent as before, make another screen capture, this time showing that the coastlines are aligned. Because the two datasets overlay, only one line color will show in the map display area. Make sure the TOC window is included in both screen captures to show that both datasets are present. This screen capture is part of your deliverables.
H. Save the map document
That is it for Part IV
You have just completed Part IV of this project, which involved some pretty important stuff. Not only do you need a handle on the map projection, but you need to be highly aware of the importance of being able to guide datum-level transformation.
The deliverables from this Part of the lesson will be:
- your puerto_rico.mdb geodatabase file.
- the two screen captures from Sections F. and G., showing the datasets with and without the datum adjustment. Save these images with appropriately descriptive names.
- a brief comment on to what extent does a datum impact map scale. Discuss this in your deliverables as a word file.