Sea Level Elevation Files
Q: When I draw a path profile along an ocean coastline, sometimes there are gaps or missing data points along the profile. How can I correct this?
A: With TAP6.0.2177 or later and a Maintenance Subscription date of August 31, 2007, or later, the Topo Utilities module includes the ability to create topographic elevation files (in the .BIL format) for a rectangular area, setting all elevation values to zero.
Note that all values in this example are for demonstration purposes only. You should determine appropriate settings for your application.
It is your responsibility to be sure that the area for the files you create is all at the elevation of mean sea level, and that no land, islands, etc., exist in the area, since these will not be included in the zero-elevation files.
Many areas that are completely ocean are not included in topographic elevation files, such as the SRTM data. This function can be used to create simulated elevation files with a fixed elevation value of 0MSL.
For example, suppose you have a path profile drawn as shown, with no elevations available in the region shown.
(Note, for the purposes of this illustration, the path between the hypothetical sites has been drawn with a non-standard earth-curvature correction and is not typical. Most paths this long would show more exaggerated earth curvature. Since the point of this illustration relates to the missing data, the earth curvature was changed for the example.)
There are other reasons for missing profile segments, for example, if you do not have data files for part of the area. You should verify that the area in question is over water and that no land exists in the area. Unless you are certain that the section is entirely ocean, you should not use this function.
You can determine the coordinates of the missing area using the Mouse Information Window. You will use the coordinates to specify the area for the file to be created.
Once you know the coordinates of the region of missing data, close HDPath. After you create the file and index it as described below you will re-start HDPath in order to use the new data.
From the TAP6 Configuration menu, select the Topo Utilities item, and then click “Create Zero-Elevation BIL File”
When the Create Zero Elevation BIL File form is displayed, enter the latitude and longitude range for the rectangular area file to be created:
Enter a file name to create (or use the “…” browse button to navigate to the desired folder). It may be helpful to create a “ZERO ELEVATION” folder under the DATA folder where TAP is installed. It is recommended you assign file names based on the southwest corner of the area, since this is a common convention in several data types.
The file name can be anything you want, but using a naming convention will help you keep track of the files you create.
To create the file, click the Continue button.
When the file has been created, click OK to exit the function.
The new file must be added to the TAP TopoScript Elevation index, just like any other topographic data you add to your TAP system. In the Configuration menu of TAP, click Topo Data:
On the Index Topographic Data Files form, click the “Browse folder to index topo data files” button:
Browse to the folder where you created the new file, and click OK:
Now when you draw the same profile, the missing data has been supplied by the new file.
The simulated elevation files that are created use an extremely low value for horizontal resolution (1-arcminute, or 60-arcseconds). If you have, or later you add other, legitimate topographic elevation data files that overlap the area of the simulated files, TAP will use the legitimate data (assuming it is a higher resolution, such as 1-arcsecond. This is because TAP selects the highest horizontal resolution data (the most detail) when reading multiple data files.
 |
Copyright 2008 by SoftWright LLC