Flight Unlimited III high resolution terrain processing workshop
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How to use srtm-1 high resolution elevation data for FU3
Introduction
The SRTM
data sets result from a collaborative effort by the National Aeronautics and
Space Administration (NASA) and the National Imagery and Mapping Agency (NIMA),
as well as the participation of the German and Italian space agencies, to
generate a near-global digital elevation model (DEM) of the Earth using radar
interferometry. The SRTM instrument consisted of the Space borne Imaging
Radar-C (SIR-C) hardware set modified with a Space Station-derived mast and
additional antennae to form an interferometer with a 60 meter long baseline.
The DEM
file (.hgt) is provided as 16-bit
signed integer data in a simple binary raster. There are no header or trailer
bytes embedded in the file. The data are stored in row major order (all the data for row 1, followed by all the data
for row 2, etc.). All elevations are in meters referenced to the WGS84 EGM96
geoid as documented at http://www.nima.mil/GandG/wgsegm/.
Byte order is Motorola ("big-endian") standard with the most
significant byte first. Since they are signed integers elevations can range
from –32767 to 32767 meters, encompassing the range of elevation to be found on
the Earth. In these preliminary data there commonly will be data voids from a
number of causes such as shadowing, phase unwrapping anomalies, or other radar-specific
causes. Voids are flagged with the value -32768.
the srtm-1
data from the nasa project are available at ftp://e0mss21u.ecs.nasa.gov/srtm/
the you
will see a long list of zip files ordered by lat / long coordinates of the
bottom left corner ( N45E005.hgt.zip for 45north/05east ). Select the zip files
of the area of interest
SRTM-1: 30m
grid at equator, datamatrix 3601 x 3601
SRTM-3: 90m
grid at equator, datamatrix 1201 x 1201
For FU3
computation we need Intel data format. for a decimal value of 26 we have 0x1a
hex in Motorola format: “00 1a” and in Intel format: “1a 00” and remember world size of 1 FU3 megatile is 16384m
x 16384m. You will have to crop an area of this size or multiple of it.
The
conversion is made using an excellent and easy to use tool DEM2BSC.EXE.
you will
find this freeware at: http://www.ptsim.com/downloads/DEM2BSQ.ZIP look for DEM2BSQ.ZIP
Preparation of the needed
files
You can download GeoTilesViewer v3.1 122KB right here.
You are using this freeware tool at your own responsibility !
- Download the .hgt.zip files of the
area you want and unzip the .hgt file to a work folder
- place a copy of DEM2BSQ.exe in this
work folder and run it.
- Select Convert HGT Cell and select
the .hgt file, the new file is named .bsq
- Now you can view the data by choosing View
a BSQ Cell or Convert BSQ to Bitmap.
- You can crop the bitmap and reload it to a
bsq file using Convert Bitmap to BSQ
- Merge a BSQ Cell and Crop a BSQ Cell until
you get the final megatile area or multiple of it (6 x 5)
- Eliminate the voids (-32767) using BSQ
Processing and selecting the options
- Upsize to 1 arc using Up/Down Sample
BSQ.
- View the final BSQ Cell and note the rows/columns
values
- Close DEM2BSQ and start GeoTilesViewer 3.1
- Choose Terrain Export enter filename,
rows/columns of bsq area and corresponding megatiles have BSQ selected and
click on Convert this will create the elevation map for FU3 in tga format.
- Starting from the bottom left corner cut
257x257 squares and preserve the overlap of top row / right column as start position for the next
square !
- Import all the named square (e0xxyy.tga)
using GeoTilesViewer.
- The water areas are not flat for this
unedited srtm data so you must flatten lakes or sea parts. You can do this
by painting the water area in the e0xxxx.tga with a unique colour of the elevation.
Generate the colourscale from the script section of GeoTilesViewer to
select the elevation wanted zoom
in the result file es2000.tga. never interpolate by graphic method
! Results are unpredictable.. An other way is to export a part of the
megatile elevation as MD3 file ready to use with Gmax, see tutorial.
Adjust lake border so water is not on the hill and import it back from MD3
format. Look at MD3 Tutorial how to do this.
In this tutorial we will
use the first 30 megatiles of the Switzerland region
The megatile range is from bottom
left aaaa to afae so we have a rectangle of 6 x 5 megatiles.
Now we have to find the
coordinates for this area of 98.304km by 81.92km use a map or any geographic tools for
that. (Size = 6*16.384km x 5*16.384km)
For this coverage we need 6
.bsq files: N45E005.BSQ, N45E006.BSQ, N45E007.BSQ, N46E005.BSQ, N46E006.BSQ, N46E007.BSQ.
Using Merge and Crop we put all together and applied the final crop for the
exact area limit by trial and error. Process and expand to 1 arc second then
save the large result file (23MB) as aafec.bsq.
The BSQ parameters for
aafec.bsq are: 2660 rows and 4576 columns.
Open GeoTilesViewer Terrain
export section and enter the values for the BSQ process, then convert.
The new Output file is
aafecBSQ.tga using the FU3 elevation scale. The size of this file is set for
the 6x5 megatile elevation file, 5*256+257 = 1537 wide and 4*256+257 = 1281 height.
A single square with
partial view of the lake of Geneva before and after correction of the lake
altitude.
Real picture and FU3 view of Emosson lake
.
under construction last update 14.aug.2005
Information: André Meystre
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