A new data frame has no coordinate system. When you add your first layer to it, the data frame takes on that layer's coordinate system. For any subsequent layers that you add, one of two things happens. If the new layer's coordinate system already matches the data frame, the layer is added without question. If the new layer's coordinate system is different, you get a GCS warning and ArcMap changes the coordinate system of the new layer to match the existing one. This operation is called a datum transformation (or coordinate system transformation). For all new layers that you add, ArcMap does these transformations automatically so that layers with different GCSs can be displayed together. (The transformations are done "on the fly," meaning they are applied only inside the data frame; the GCSs of the data sets on disk are not changed.)

It's great that ArcMap does datum transformations for you, but complications arise because there may be multiple versions of a transformation. For example, ArcMap has seven formulas that convert from NAD27 to NAD83. Each works best for a particular region.

Similarly, there are 21 different transformations from NAD27 to WGS84.

For each datum transformation, ArcMap has to pick a default formula. For the NAD27 to NAD83 transformation, it uses NAD_1927_To_NAD_1983_NADCON, which is optimized for the continental United States. If your data happens to lie in Alaska or Canada, your features may still not line up properly until you override the default transformation and pick the optimal one.

The default NAD27 to WGS84 transformation is NAD_1927_To_WGS_1984_1, which is the optimal transformation only for Caribbean data.

In this exercise, you will see how ArcMap applies its default datum transformations and you will learn how to change these settings.

Estimated time to complete: 45 minutes

Before you begin The data for this exercise is contained in the following files: Datums.exe

Step 1 Start ArcMap and add a layer

Start ArcMap and open a new map document.

Click the Add Data button, navigate to the TransformCS folder within your module folder (e.g., C:\Temp\VirtualCampus\MapProjections\Datums\TransformCS) and add Streets.shp.

Step 2 Check the data frame's coordinate system

Open the Data Frame Properties dialog and click the Coordinate System tab.

The coordinate system is set to GCS_North_American_1927.

Click the Transformations button. (Depending on the version of ArcGIS software you are using, your dialog may look slightly different than the View Result graphic.)

Since the data frame has only one layer, the "Convert from" and "Into" datums are the same and no transformations are listed in the "Using" box. If you know the transformation you want, you can set it up ahead of time; otherwise, you can let ArcMap apply a default transformation.

In the next step, you'll add a layer with a NAD83 GCS and let ArcMap set the datum transformation.

Click Cancel on the GCS Transformations dialog and on the Data Frame Properties dialog.

Step 3 Add a second layer with a different GCS

Click the Add Data button. Navigate, if necessary to the TransformCS folder and click to add ParcelsNAD83GRS80.

A warning message tells you that the new layer's GCS is different from the data already in the map. It also tells you that a geographic transformation has been selected for you, but that you may need to change it to avoid alignment problems.

Click Close on the warning message.

You need to zoom in to see the parcels.

In the table of contents, right-click the parcels layer and click Zoom To Layer. That brings you a bit too close.

In the map scale box, type 8000 and press Enter.

Although the two layers have different GCSs, they align nicely. It looks like ArcMap picked the best available datum transformation. Now you'll take a look to see which one it is.

Step 4 Check the datum transformation

Open the Data Frame Properties dialog, click the Coordinate Systems tab, if necessary, then click the Transformations button.
In the GCS Transformations dialog, both GCSs found in the data frame are listed. Click GCS_North_American_1983, the GCS of the Parcels layer.

ArcMap has transformed the NAD83 datum to NAD27 using the NAD_1927_To_NAD_1983_NADCON formula. As you may have seen in the exercise introduction, this transformation is optimized for the continental United States and is therefore the best choice for your southern California data.

Click the Using dropdown arrow to see all the available transformations.

Some of them have descriptive names, but others don't. Later in this exercise, you'll learn how to use a table that helps you find the best transformation for your area of interest. You should always confirm that you are using the optimal transformation—even when you don't notice alignment problems in your data display.

Click Cancel on the GCS Transformations dialog and on the Data Frame Properties dialog.

Step 5 Add a third layer in still another GCS

Click the Add Data button. Navigate, if necessary, to the TransformCS folder and add Hydrants.

Again, click Close on the GCS warning message.

The hydrant points show up, but not in the correct locations. Each fire hydrant should lie inside a parcel and near its edge. These hydrants are shifted to the left by about four parcel lengths. What went wrong? ArcMap's default datum transformation is probably not the optimal one.

Step 6 Check the datum transformation

Open the Data Frame Properties dialog, click the Coordinate Systems tab, if necessary, and click the Transformations button.

In the Convert from pane, click GCS_WGS_1984 (the GCS of the hydrants layer). (Depending on the version of ArcGIS software you are using, your dialog may look slightly different than the View Result graphic.)

The default transformation applied by ArcMap is NAD_1927_To_WGS_1984_1. As you may have seen in the exercise introduction, this formula works best for Caribbean data, so it's not the best choice for your southern California data.
Click the Using dropdown arrow to see the other transformations.

There are about twenty. How do you know which one to use? If there were only a few, you could try them all, but with so many choices that would take a while.

Fortunately, ArcMap comes with a file called geographic_transformations.pdf that lists supported datum transformations and the areas for which they are optimized. This file has also been included in the data you downloaded for these exercises.

Step 7 Find the best transformation for your area

Open Windows Explorer.

In Windows Explorer, navigate to the TransformCS folder within your module folder (e.g., C:\Temp\VirtualCampus\MapProjections\Datums\TransformCS).

Double-click geographic_transformations.pdf to open the file.

Navigate to page 34 of the document.

At the bottom of the page begins a list of transformations for North America. The left column shows the area for which the transformation is best suited and the right lists the transformation.

Browse through the list. You can confirm that, as the previous step suggested, the NAD_1927_To_WGS_1984_1 transformation that ArcMap picked for you is used for the Caribbean. It is listed separately for the countries of Antigua, Barbados, Barbuda, Caicos Islands, Cuba, Dominican Republic, Grand Cayman, and Jamaica. No wonder your fire hydrant data is off.

Scroll down to page 37 and locate United States (contiguous states west of Mississippi River) in the left column. The transformation for this region, and the one that therefore best fits your California data, is NAD_1927_To_WGS_1984_6.

Step 8 Apply the new datum transformation

Close Adobe Reader and make ArcMap active.

In ArcMap, the Geographic Coordinate System Transformations dialog box should still be open. From the Using dropdown list, choose NAD_1927_To_WGS_1984_6.

Click OK on the GCS Transformations dialog and on the Data Frame Properties dialog.

It looks like the new transformation has fixed the problem. The fire hydrants have shifted to the right and appear to be positioned inside parcels.

But is everything really as good as it seems?

Step 9 Check your results

Click the Zoom In tool on the Tools toolbar. Zoom in on the fire hydrant in the bottom cul de sac (in the lower left corner of the parcels data). You may have to use the tool a few times to see that the hydrant is not really inside the parcel but out in the street.

Next, you'll identify how far you'd have to move the hydrant to get it back inside the parcel and off the street.

Open the Data Frame Properties dialog and click the General tab.

In the Units pane, from the Display dropdown list, choose Feet.

Click OK on the dialog and click Yes on the GCS warning.

To make the measurement you should be zoomed in really close. Set your map scale to 1:100.

On the Tools toolbar, click the Measure tool.

Click on the hydrant and then click inside the parcel to get a distance.

It is about two feet from the hydrant to the edge of the parcel. So the hydrant points are still off by that amount. (It would be nice to have a precise air photo to compare positions, but this is a new subdivision and air photos don't yet exist for it.)

Step 10 Solve the problem

Getting back to the reason for the problem, take another look at the datum transformation.

Open the Data Frame Properties dialog, click the Coordinate System tab, and click Transformations.

In the Convert From pane, click GCS_WGS_1984.

Note that the method is Geocentric Translation. This is a three-parameter transformation, which is a relatively low-accuracy transformation—especially when you're going from a local datum to an earth-centered datum (because the origins of the two coordinate systems are far apart).

An easy solution would be to change the data frame's coordinate system so that the NAD_1927_To_WGS_1984_6 transformation doesn't have to be applied. You can do this by changing the data frame's GCS from NAD27 to NAD83. What does that do for you? First, it still lets you apply the very accurate NADCON transformation to align the streets and parcels. Second, it lets you transform between WGS84 and NAD83 (instead of between WGS84 and NAD27) to align the parcels and hydrants. This is an advantage because WGS84 and NAD83 are both earth-centered datums with similar specifications; consequently, this transformation is more accurate.

Click Cancel on the GCS Transformations dialog.

Step 11 Change the coordinate system of the data frame

Make sure the Data Frame Properties dialog is open and the Coordinate System tab is selected.

In the Select a coordinate system box, open the Predefined folder, then the Geographic Coordinate Systems folder, then the North America folder.

Scroll down and click North American Datum 1983.

Click OK on the Data Frame Properties dialog. If you get a GCS warning, click Yes.

The hydrant has moved inside the parcel boundary, where it should be.

Step 12 Check your results again

Click the Measure tool, if necessary, and measure the distance from the edge of the parcel to the new location of the hydrant. It's about half a foot away.

If you like, check the locations of some other hydrants to make sure they are all within parcel boundaries.

Open the Data Frame Properties dialog, click the Coordinate System tab if necessary, and click Transformations.

In the Convert From pane, click GCS_WGS_1984. (Depending on the version of ArcGIS software you are using, your dialog may look slightly different.)

The default transformation is NAD_1983_To_WGS_1984_1. This is the appropriate transformation for all of North America.
Note that the method is still Geocentric Translation—a three-parameter transformation. The improved accuracy is due not to a more sophisticated transformation, but rather to the similarity between the NAD83 and WGS84 datums.

Click Cancel on the GCS Transformations dialog and on the Data Frame Properties dialog.

It may seem like you used a little sleight-of-hand, but all you really did was take advantage of the fact that some datum transformations work a little bit better than others. Here are a few points to remember:

Conversions between NAD27 and NAD83 are highly accurate because they use a grid-on-grid transformation method.

Conversions between NAD83 and WGS84 are accurate because both datums are earth-centered and have similar specifications.

Geocentric translations between local and earth-centered datums (like NAD27 and WGS84) may lack the desired accuracy at large scales.

Step 13 Save and close the project

From the File menu, choose Save As.
In the Save As dialog, navigate to your module MyWork folder (e.g., C:\Temp\VirtualCampus\MapProjections\Datums\MyWork). Name the file myTransform and click Save.

Close ArcMap when you are finished.