Updated NED Data
SoftWright Home
Q: Can I use updated USGS elevation data for the National Elevation Dataset (NED)?
A: With TAP6.0.2435 or later and a Maintenance Subscription date of February 29, 2012, or later, you can use the USGS data in the Gridfloat format provided by the USGS.
Note that all values in this example are for demonstration purposes only. You should determine appropriate settings for your application.
As an example of the differences between the original NED files (released in about 2000) and the files obtained in November 2011, consider a sample path (in western Utah):
The actual changes in different areas of the country will, of course, vary. But this profile provides at least one idea of the effect of the USGS updates to the topographic elevation data files. Maps showing the areas where the data files have been updated are shown below. The .FLT data can be purchased in approximately the same area blocks used by the older .BIL files. Since the .FLT format files from the USGS fall on even degree lines of latitude and longitude, the blocks for the .FLT files will often extend beyond the more arbitrary borders of the .BIL files (the same block will often include more data in the .FLT format).
If you have not used the older NED files in the .BIL format you can copy the new files in the .FLT format to the folder where you want to use them, run the TAP Topo Data Index function, and begin using the files immediately. It is strongly suggested you put the .FLT files into a dedicated folder, such as a folder named “FLT Files.” Having the files in a folder by themselves will make managing the files easier if you later add other types of topographic elevation data.
However, if you have previously used the .BIL files you will need to follow the procedure described here.
The NED files provided by SoftWright in both the .BIL and the .FLT formats are “1-second” resolution data, meaning that the sample elevation points contained in the file are approximately 30-meters apart (depending on the geographical location of the area contained in the file).
By default TAP searches the topo data index for an area in order to find the highest resolution topographic elevation data to use. Since both .BIL and .FLT files are the same resolution, the possible ambiguity would mean that TAP would not necessarily select the updated .FLT files instead of the older .BIL files.
Because of this, you will need to separate the updated files from the older files. There are two ways you can do this.
· If you are not interested in preserving the older .BIL files you can simply delete them from your TAP system. The files are typically in the DATA folder where TAP is installed and usually will be in folders with names such as SWNED0001. You can delete these folders, copy the new .FLT files to the desired folder, and run the Re-Index function described below.
· On the other hand, you may want to keep the .BIL files available. This might be the case if you want to do any comparisons between the old and the updated data files. If you may have a need to recreate previous work that used the .BIL files, you can keep both the .BIL files and the updated .FLT files on the machine, but in separate folders. To avoid the ambiguity mentioned above you will need to use the Topo Data Selection function in TAP to select either the .BIL format or the .FLT format. You will also need to make the appropriate change to the configuration for site elevation data:
For normal operation, using the updated .FLT files, you would select the configuration as shown. For the testing or other functions you could un-check the .FLT files and check the .BIL files.
After adding the .FLT files to your TAP system (in either scenario described above) you will need to run the Re-Index Topo Data function.
The Topo Data Index is used by the software to manage the topographic elevation data files. To add the updated .FLT files to your index, click the Configuration menu in TAP and then click “Topo Data”
Since ReIndex function will remove references to any deleted or removed files (such as any of the older .BIL format files) and create a new index. Click the “ReIndex Topo Data Files” button on the form:
A warning message is displayed. Click the Yes button to continue.
You will have the option to save the existing index. In this case, especially if you have removed the .BIL files, this is not needed. Click the No button to continue:
Navigate to the folder where the .FLT files are installed and click the OK button to continue.
If the .FLT files are on an external USB disk drive, select the root directory of that drive.
The files will be added to the index:
Copyright 2011 by SoftWright LLC