Using WMS to Perform Coordinate Conversions

Introduction

WMS can convert data residing in the graphics window from one coordinate system to another.  Note that the coordinate conversion is performed on the temporary data being manipulated by WMS, and that the original files are not converted.  However, if desired, a user can save or export a new file from WMS that will be based on the new coordinate system.  

A common reason users might need to convert data from one system to another is that having obtained two geographic data files for their area, they realize that each file is based on a different coordinate system.  The user wants to open both files in WMS and have the two files line up correctly.  This is a simple conversion, because only two files are involved.  The user would open one of the files in WMS, convert it to the second file's system, and then import the second file.  

The user might have three or more files, each in a different system.  In this case, the user needs to decide on a common coordinate system to use, individually convert any files not already in the common coordinate system, and save or export out a new file in the new coordinate system.  Finally, the user opens all the files with the same coordinate system into WMS.  As mentioned, any files already in the common coordinate system will not need to be transformed beforehand.

EXAMPLE

To illustrate the steps involved in a simple coordinate conversion within WMS, I have outlined the steps I took to convert a Digital Elevation Model (DEM) from UTM NAD 1927 to UTM NAD 1983.  

Problem

The MSN TerraServer^® web site offers topographic map and aerial photo images based on the UTM NAD 1983 Coordinate System.  However, DEM files provided by WebGIS^® are in UTM NAD 1927 coordinates.  I downloaded an image from TerraServer and a DEM from WebGIS and want to view both files simultaneously within WMS.   I also want the image and DEM to overlap geographically.  For this example, I chose to convert the DEM (in NAD 27) to the Topo Map's coordinate system (NAD 83), but could have converted the topo map image instead.  See the note on converting image data to another coordinate system.

The following figure shows what the data looked like when I imported both files into WMS without performing any coordinate conversions.  Notice that the red DEM contour line representing the edge of the bay does not quite line up correctly with the bay's outline on the topographic map.

                
                Figure 1:  The DEM contour depicting the edge of Anchor Bay is in red.

Solution

Steps to convert the DEM from UTM Nad 27 to UTM Nad 83:

1. Open WMS and import the DEM file(s),
2. Select Coordinate Conversion... from the Edit menu,
3. Click the Current Options button at the upper-left part of the Coordinate Conversion dialog,
4. Select "UTM NAD 27 (US)" from the Horizontal System drop-down list,
5. Select "17 84W to 78W" from the UTM Zone list (this is the UTM zone corresponding to this example area),
6. Select "meters" from the Units list,
7. Leave the Vertical System at "Local" (always check your metadata for help in selecting the appropriate vertical and horizontal systems, and units),
8. Change the vertical Units to "meters" (this DEM is in meters both horizontally and vertically),
9. Click OK to exit the Coordinates dialog,
10. Change the Horizontal System to "UTM NAD 83 (US),"
11. Change the other fields to match steps 5 through 8 above, since we're only changing Horizontal Systems,
12. Click Convert to exit the Coordinate Conversion dialog and begin the coordinate conversion,
13. Import the Topo Map image, which is already in the UTM NAD 83 (US) coordinates.

Notice in Figure 2 that the DEM and image overlap much more accurately within the WMS graphics window:

               
                Figure 2:  The bay's DEM contour line after performing the coordinate conversion.

Additional Notes on Coordinate Conversions in WMS

Creating a File Based on the New Coordinate System

After having converted the DEM data to a new coordinate system, if a user wanted to maintain the DEM in the new coordinate system, she could export the data from WMS in the following manner:

1. Select Export from the File menu,
2. Choose "Arc/Info Grid (*.asc)" from the Save as Type drop-down list,
3. Name the file and click Save.

Now, when the this *.asc DEM file is imported into WMS, it will be in the new coordinate system.

Coordinate Conversions on Images

Now let's suppose a user has used WMS to transform an image to a new coordinate system, but he wants the ability to open the image later in WMS without having to perform the conversion again.  To accomplish this, the user can save the image as a WMS Image file (*.img) in the following manner:

1. Select Save As... from the File menu
2. Choose "Image Files (*.img)" from the Save as Type list,
3. Name the file and click Save.

The *.img file contains information for registering an image in its body.  By saving the *.img file after converting coordinates, the registration information acts to transform the original image to that coordinate system whenever WMS opens the *.img file.  Note that the *img file is not the image itself, rather, it "calls" in an image file and automatically registers it.  The *.img file is native to WMS.  Currently, WMS does not support the exporting of a geo-referenced image.