TruFlite Short Manual - Data Import, SRTM Data

TruFlite for Windows Workshop - www.truflite.com

Import

Data import, SRTM data

Under agreement with NASA, the USGS EROS Data Center distributes and archives SRTM data in accordance with a joint partnership Memorandum of Understanding between NASA and NIMA (July, 2002).

SRTM data can be obtained via the USGS Seamless Data Server Distribution System - Enhanced.

Let us retrieve the one by one degree area around San Francisco:

	38°N
123°W		122°W
	37°N

Let us start at their home page:

http://seamless.usgs.gov/

View and Order Data Sets - United States Viewer: http://seamless.usgs.gov/start.htm

Start page of the SDDS Raster Extraction Website

To the right, select "Download Layers/Raster/SRTM 30m".

At the left margin, follow the "Define Area By Coordinates" link. Switch to decimal degrees and enter the desired coordinates:

Hard copy: Limits

Click "Add Area".

You'll be asked this:

Clicking "OK" yields this result:

Data Extraction Request Pieces:
Product	Southwest (Bottom Left) Corner	Northeast (Top Right) Corner	Size (MB)	Link
SRTM - ArcGrid format
	37.000000 N, 123.000000 W	38.000000 N, 122.000000 W	54	Download

Before clicking "Download", you have to modify the request: We need BIL the format.

SDDS-Enhanced Request Options Page:
Requested Product(s):	select only "SRTM 30m - Shuttle Radar Topography Mission" Data Format: change the selection to "BIL". Archive Format: ZIP Metadata Format: HTML
Breakup of request into pieces:	Maximum size per piece: 100 MB
Display Options:	WGS84 Coordinate Display: Decimal Degrees
Media Options:	Order this request on Media? No

Click "Save Changes & Return to Summary"

Data Extraction Request Pieces:
Product	Southwest (Bottom Left) Corner	Northeast (Top Right) Corner	Size (MB)	Link
SRTM - BIL format
	37.000000 N, 123.000000 W	38.000000 N, 122.000000 W	26	Download

Click "Download". A new browser window opens:

Current order status
Adding your request to the queue.../Extracting data.../Creating the archive file...
Please wait for the data to be returned.

The dialog to save the resulting ZIP file should open automatically. Choose an appropriate location and name for the ZIP file, e. g. 85311761.ZIP.

Contents of the ZIP file:

Archive: 85311761.ZIP
Length Date Time Name
-------- ---- ---- ----
25927200 06-24-03 10:47 85311761/85311761.BIL
     222 06-24-03 10:47 85311761/85311761.BLW
     180 06-24-03 10:47 85311761/85311761.HDR
    4099 06-24-03 10:47 85311761/85311761.PRJ
      22 06-24-03 10:47 85311761/85311761.STX
   20537 06-24-03 10:47 85311761/METADA~1.HTM
-------- -------
25952260 6 files

We need to extract the BIL and the HDR file.

Now, let us run TruFlite.exe:

Calculate/Import Filters/Binary DEMs...

From File: *.BIL

Header File: *.HDR

At this point TruFlite complains:

Problem when using the standard HDR file:

Though the error message is not quite correct, it indicates that the HDR file does not contain all required information.

This error message may not appear in future versions of TruFlite for Windows - we are currently evaluating pros and cons.

For the time being, edit the HDR file as follows:

before manual editing	after manual editing
BYTEORDER I LAYOUT BIL NROWS 3600 NCOLS 3601 NBANDS 1 NBITS 16 BANDROWBYTES 7202 TOTALROWBYTES 7202 BANDGAPBYTES 0	BYTEORDER I LAYOUT BIL NROWS 3600 NCOLS 3601 NBANDS 1 NBITS 16 BANDROWBYTES 7202 TOTALROWBYTES 7202 BANDGAPBYTES 0 ULXMAP -123 ULYMAP 38 XDIM 0.00027777777778 YDIM 0.00027777777778

before manual editing

after manual editing

BYTEORDER I
LAYOUT BIL
NROWS 3600
NCOLS 3601
NBANDS 1
NBITS 16
BANDROWBYTES 7202
TOTALROWBYTES 7202
BANDGAPBYTES 0

BYTEORDER I
LAYOUT BIL
NROWS 3600
NCOLS 3601
NBANDS 1
NBITS 16
BANDROWBYTES 7202
TOTALROWBYTES 7202
BANDGAPBYTES 0

ULXMAP -123
ULYMAP 38
XDIM 0.00027777777778
YDIM 0.00027777777778

The ULXMAP value is the westernmost longitude of the data, the ULYMAP value is the northernmost latitude. The XDIM/YDIM values are the extent. All values are measured in degrees.

TruFlite data import dialog, all data entered	Result
In the above dialog, we leave the default value of 256 for the z scale. Thus, the unit of the output file is 1/256th of the original. Why are we doing this? Because the elevations can be interpolated much better if the units are small. The TruFlite TGA file bandwidth allows for 16 million distinct elevation values. The factor 256 does not cause any rounding errors, so the full information is transferred into the TGA file.
This indicates -120 to 996 elevation units. The PRJ file contains the unit description: units METERS It is a good idea to use 1,000 as the z Scale: The new unit would then be millimeters.	In this original size section of the above picture, see how beautifully the waves are coming in!

TruFlite data import dialog, all data entered

Result

In the above dialog, we leave the default value of 256 for the z scale. Thus, the unit of the output file is 1/256th of the original.

Why are we doing this? Because the elevations can be interpolated much better if the units are small. The TruFlite TGA file bandwidth allows for 16 million distinct elevation values. The factor 256 does not cause any rounding errors, so the full information is transferred into the TGA file.

This indicates -120 to 996 elevation units.

The PRJ file contains the unit description: units METERS

It is a good idea to use 1,000 as the z Scale: The new unit would then be millimeters.

In this original size section of the above picture, see how beautifully the waves are coming in!

To get an idea of the wave height during the data acquisition, we searched for minima and maxima in the TGA file:

R/G/B: 255/248/1 (occurs very scarcely) is -8 meters;

R/G/B: 0/2/0 is +2 meters; +4 meters occur, but only once or twice, so maybe they hit a boat.

Basically, the average elevation seems to be less than zero meters in the raw data... We may work out a histogram of the interesting regions some day.