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Issue 60

How to Convert the Projection of a Map in Surfer 9

There is a new feature in Surfer 9 that allows the user to convert the projection of a dataset from within Surfer's worksheet window. This tool has proven to be extremely useful to users who incorporate data from multiple sources and would like all of their maps and data to be in the same coordinate system. This new Surfer feature allows for complete data integration. Surfer has more than 1500 pre-defined coordinate systems to choose from, including the most widely used systems in the industry. There is also option to reference coordinate systems that are custom defined by the user.

In order to provide a good, working example of how to convert projections in Surfer 9, a free Digital Elevation Model (DEM) of Honolulu was downloaded from the USGS in UTM coordinates. Although this article uses a DEM file, you can use any grid file. This article will show how to create a map of the original DEM file and then walk through the steps necessary to convert the coordinates of the map from UTM to latitude and longitude. If you simply want to convert the data in an XYZ data file, without starting from a map or grid file first, you can start at Step 3.

Step 1. Display the Original DEM:

It is a good idea to display your data before any conversion process is started to get a feeling of the coordinate units displayed on the X and Y axes. To display the DEM, a contour map and 3D surface layer are generated.

1. Make sure that the DEM file is unzipped into a folder somewhere on your computer.
2. Open a new plot document in Surfer by using the File | New | Plot command.
3. To create a contour map, go to Map | New | Contour Map, navigate to the DEM, and click Open.
4. To add a 3D surface layer, make sure that the contour map is selected and use the Map | Add | 3D Surface Layercommand, select the DEM file and click Open.

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A USGS DEM of Honolulu is displayed as a 
3D surface and contour map in UTM coordinates.

Step 2. Convert the DEM to an XYZ Data File:

The coordinate conversion process only works on XYZ data in Surfers worksheet window. If you don't have the source XYZ data file for the grid you are working with, you must convert the grid to an XYZ data file. This is easily done in Surfer, the step are as follows:

1. Go to Grid | Convert.
2. In the Open Grid dialog, select the grid or DEM file you wish to convert.
3. In the Grid Info section, note the size of the file in the number of rows and columns. Click Open.

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Note the Size of the selected grid file in the 
Grid Info section of the Open Grid dialog.
4. In the Save Grid As dialog , navigate to where you want to save the data file, give the file a name, change the Save as type to DAT XYZ, and click Save.

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Use the Grid | Convert command to convert your grid or DEM file to a DAT XYZ data file.

Step 3. Convert the Coordinates in the Surfer Worksheet:

The coordinate conversion process takes place in Surfer's worksheet window. To convert the coordinates, follow these steps:

1. Go to File | Open, select the DAT file previously created (or any other XYZ data file you have) and click Open.
2. You may need to edit the data before the coordinate conversion command is executed. For example, if there are blanking values present in the data, you should remove them. To do so, select all three columns and use the Data | Sort command. The DEM used in this example had a blanking value of -32767, so the option of ascending sort byColumn C works well. Then simply highlight the records that contain that particular blanking value in Column C and go to Edit | Delete to remove them.
3. Now you are ready to convert the coordinates. Go to Data | New Projected Coordinates.
4. In the New Projected Coordinates dialog, the X Source Column and Y Source Column must be assigned the corresponding X and Y columns in the data. In this case, the source X and Y columns are columns A and B.
5. Click the (...) button to the right of Source Coordinate System to enter the source coordinate system for the data.

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 Click the (...) button next to Source Coordinate System.
6. In the Assign Projection dialog, open the Projected Systems | UTM | North America section.
7. Select North America NAD27 UTM zone 04N and click OK.

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In the Assign Projection dialog, select the source coordinate system for the data, which in this case is 
UTM 1927 Zone 4N, and click OK.
8. When you click OK in the Assign Projection dialog, the selected coordinate system is entered as the Source Coordinate System.
9. Designate the X and Y Target Columns where you want the re-projected latitude and longitude data to be entered. By default, the first two empty data columns are entered.
10. Click the (...) button to the right of Target Coordinate System to enter the target coordinate system, which is latitude and longitude.
11. In the Assign Projection dialog, open the Geographic (lat/long) category.
12. Under this category many different datums are listed. You can choose which datum you want your lat/long coordinates use. Most simple lat/long coordinates use World Geodetic System 1984 (WGS84) as the datum. Select the datum you wish and click OK. This coordinate system is entered as the Target Coordinate System.

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In the New Projected Coordinates dialog, specify the Source and Target
Columns and the Source and Target Coordinate Systems.
13. Click OK and the coordinates in the designated source columns are converted and entered into the designated target columns. Use the File | Save or File | Save As commands to save the data.

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File | Save or File | Save As commands to save the data.  
The original UTM X and Y data in columns A and B are
converted to longitude and latitude data in Columns D and E.

Step 4. Grid the Converted Data and Display the Map in the New Coordinates

1. To display the converted XYZ DEM information, the data must be re-gridded. Go to the plot window and use the Grid | Data command.
2. Select the data file saved above and click Open.
3. In the Grid Data dialog, select the new longitude and latitude columns for the X and Y columns. In this example, columns D and E are selected.
4. Select Nearest Neighbor for the Gridding method and change the # of lines to be the number of columns (X) and rows (Y) noted above when you converted the original grid or DEM file. Click OK.

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Go to Grid | Data to grid the converted data using the Nearest Neighbor
gridding method and the same number of rows and columns (grid lines) as
the original grid or DEM file.

The result is a grid file that has a different coordinate system than the original DEM. You can create a contour map and a 3D surface layer, similar to the first image in this article. You will notice that the axis on the map now displays the converted latitude and longitude coordinate units.

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The map of Honolulu is now displayed in latitude and longitude coordinates.
 

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May 17, 2012

by drew


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