Sign in to follow this  
Guest RobStan

image projection to WGS84

Recommended Posts

Hi all,I am following the 'Creating Custom Terrain Textures' manual from microsoft FS2002 SDK, I am trying to ad my photo texture to an area, the instructions say...'it is assumed that the image has been projected to the WGS84 Lat/Lon datum'....what does this mean? do i have to have the edge of the image set to a specific Lat/Lon area?Thanks in advance, Rob.(-:

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

This is a good question when dealing with elevetion mesh! You really have to get this down if you want your mesh to be horizontally correct. In other words, the mountain is in the correct place.Projected means that the coordinate projection used is of the WGS84 Lat/Lon datum. So what does that mean? First of all what is a coordinate projection? A projection is how the spherical earth is placed on a flat piece of paper. There are hundreds of different projections in use today. Most countries have their own standard for projections of their land. A good site to learn about projections is:http://www.colorado.edu/geography/gcraft/n.../mapproj_f.htmlIn 1984, a new standard came out that could be used worldwide. It is called the WGS84 projection standard. It came about to facilitate the use of GPS software since each projection is just a little bit off from others. Thankfully, FS uses this standard too.Lat/Lon datum just means that the data is lined up along a specific latitude and longitude line.The real problem is getting good elevated mesh in this format. A real good place for U.S. data is http://seamless.usgs.gov. The format can be downloaded as WGS84 Lat/Lon datum.Lee Steffensen.

Share this post


Link to post
Share on other sites

Howdy:Lee, thanks for the great summary on projections and information sources!As for georeferencing of photoreal images: below is a draft (!) step-by-step guide how to reproject an airphoto or satellite image (in this case a TerraServer DOQ) from UTM/NAD83 or similar projections to lat-long/WGS84 using freeware only. Please read all the way to the end as the last few steps are a bit 'shaky'. I came up with this procedure at someone elses request; however, I have not fully tested it because I have (payware) GIS software that is faster and more straightforward.If anyone knows of a better/quicker method, please let us know. Also, let me know if you find a mistakein or problem with my procedure.I know that there are alternative methods with graphics software like Photoshop that use graphical stretching or 'rubbersheeting'. However, if the photo area is fairly large a 'formal' geographical procedure will give more accurate results. Cheers, Holger----1. Download and install MicroDEM (full install) and MapMaker Gratishttp://www.usna.edu/Users/oceano/pguth/website/microdem.htmhttp://www.mapmaker.com/index.htm2. Set up a project folder3. Move your DOQ .jpg file into the project folder and convert to 8-bit grayscale bitmap (.bmp) with Photoshop, PSP, or other graphics software.4. Make sure you have a record of the four corner coordinates in UTM/NAD83, the image resolution (e.g., 4 meters), and the UTM zone (e.g. 10)5. In Mapmaker, use "Utilities > Bitmap Utilities > Convert File Format > from BMP" to open 8-bit DOQ .bmp and save in Idrisi raster file format (.rst). Mapmaker will create the raster file and an associated definition file (.rdc).6. Open the .rdc file in Wordpad and overwrite the corner coordinates (min/max X and Y) with the correct ones for your DOQ. Also, overwrite the ref.system parameter to "us93tmXX", with XX being the UTM zone (e.g., us83tm10 or us83tm8). Then check that ref.units is set to "m" (meters) and unit dist. to the source resolution of your image (e.g., 4.0). An example for an area in Washington State looks like this:ref. system : us83tm10ref. units : munit dist : 4.0000000min. X : 5353600.0000000max. X : 5356000.0000000min. Y : 523200.0000000max. Y : 526400.0000000Save the .rdc file (all other fields can stay as they are).7. In Mapmaker, use "Utilities > 3D surface utilities > Import surface > Idrisi (*.rst)". Choose the .rst file, the output .ter file will have the same name. Click on "Finish". The preview of your georeferenced .bmp file is a good check for problems.8. In Mapmaker, use "Utilities > 3D surface utilities > Export surface > ArcGrid (*.grd)". Choose the .ter file and confirm save.9. In your project folder, rename the .grd file to .asc (otherwise MicroDEM won't "see" it).10. In MicroDEM, use "File > Data Manipulation > Import > ASCII DEM > ASCII Arc Grid" to select your .asc file. Enter the correct UTM zone, and keep the Z multiplier as "1". In the Header window, double-check the entries and change "Digitizing Datum for DEM" to "NAD83". Click on "OK", then "No" on "Feet input". The .DEM file will be generated as a preview and you're asked to "Click to close new DEM". Cancel the next "Open DEM grid" window and return to the Data Manipulation window.11. Select "Resample > DEM > DEM as lat/long grid" and choose your new .dem file. Enter an appropriate spacing for the resampling procedure; 1 sec equals approx. 30 meters, 0.1 sec is approx. 3 m, 0.2approx. 6 m, etc. MicroDEM will interpolate and ask you for a new name for the resampled .dem file.12. Close the Data Manipulation window, then use "File > Open DEM" to display the resampled DEM. Since MicroDEM has limited export capabilities, I use the "save image" function. First, select 1:1 view from the toolbar (the magnifying glass with the 1 inside) and make sure that your display parameter is "elevation" and the colors are "Gray scale" with ocean and lake checks unchecked and without grid or labels (all accessible via right mouse click). Now, use right mouse click to select "Save image", browse to your project folder, and choose a name for your image (use either .bmp or .jpg (deliberate, with compression factor of 100). 13. The resampled image is now ready for further manipulation in your graphics program.---Note!!! I'm not sure about Step 12. MicroDEM automatically converts and displays any DEM in the chosen Primary Datum (selectable in Options > Maps; search the help file for "datum issues" for further explanations). This probably explains why the input UTM/NAD83 and output lat-long/WGS84 appear so similar. The problem is that we're taking a screen shot rather than exporting the converted file and thus have to assume that the display is in fact lat-long/WGS84. Since a conversion of this type usually results in a major distortion of the x-axis (see the help files of 3DEM (freeware, at http://www.visualizationsoftware.com/) for a visual example), I have the suspicion that the visual output of MicroDEM is not in lat-long and thus the screen capture a UTM/WGS84 rather than a lat-long/WGS84. I'll have to check that.

Share this post


Link to post
Share on other sites

Hi guys, thanks for the input, much appreciated. I would have thanked you sooner but had a slight system problem. I will follow up on your suggestions. It looks like head down etc..etc... for a while & I should get the swing of it. Now to find some decent sources for Australian data for download...........Thanks again. Rob.:-)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this