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  • Kevin
    Kevin

    Maps: The Sea Level Problem with USGS Data

    Please PM STomnibus if you have any questions about this article.

    This is in regard to Rybolton's article on using USGS Data to make terrainsin Sim City. I have been experimenting with it and have found that thereare some improvements that can be made.The problem comes at the point where the data is being manipulated withMICRODEM after the image has been converted to gray scale.

    At this point,Rybolton chooses "Specify" from the "Colors" menu and raises the terrain bysubtracting about half of the difference between the highest and lowestpoints from the lowest point. This effectively brings "sea level" (in theSim City model) to about where it actually would be. However, this has theunwanted side effect of reducing the apparent topography of the final regionby about 1/2. This is my solution:Whatever region you pick, MICRODEM uses the full gray-scale range from whiteto black when converting to gray scale: Your highest elevation will bewhite, your lowest, black. On coastlines, the lowest elevation is sealevel.

    Inland, you may have a valley with a lake at the lowest point or ariver. If you want your lowest point to be submerged in the final region,the best way to do it is to apply as little correction as possible to thegray scale map and then use the terrain tool to lift each city inside thefinal region by a given amount.After converting to gray scale in MICRODEM, select "Elevation Colors" and"specify". A dialog box appears with the lowest elevation on your map. Notethis number but do not change it. Press "Next" and the highest elevationappears.

    Again make note and press "Next". The map will redraw exactly asit was before, but wasn't that fun.Now, subtract the low elevation from the high elevation and multiply theresult by 0.067 (in my experiments I have found that the final value isbetween 6% and 7%, usually closer to 7%.) Subtract the resulting numberfrom the low elevation and write it down.Now go back to "Elevation Colors" and "Specify" again. In the firstdialogue box, change the low elevation to this new number. Press "Next".Do not change the high elevation, just press "Next" again.Now, finish making your Sim City region as per Rybolton. When the regionfirst appears, it will be mostly submerged.

    Now comes the most tediouspart: Each city in the region must be entered separately and the TerrainLift Tool must be applied 5 times to each city. The faster your computer,the less you'll hate this. You should end up with water filling a shallowbasin in your lowest areas. This will probably need to be deepened usingthe appropriate Terrain Tools.Here's the rub: This method is approximate. It will get you to within 10meters of the proper water level; but 10 meters can leave you with no waterat all or with a flood.

    My suggestion is to make a region 4K X 4K (the sizeof a large city). (See the help on creating custom sized regions andconfig.bmp.) Then, when you make your 1025 X 1025 pixel .jpg file, alsomake one that is 257 pixels square. Use this in your small region as atest. If the water level doesn't turn out right, go back and try again.Subtracting another meter or three will raise the terrain, subtracting lessthan you originally did will lower the terrain. It goes pretty fast whenusing a test region. When the test region looks good to you, then go backto the corresponding large .jpg file and make your final region.One last note: Once in a while the final .jpg file won't work even thoughit is the right size.

    In some cases I have found it is better to make a.bmp file and use it; however the same thing can still happen. Changing thecompression of the .jpg file also sometimes fixes the problem. Weird

    .--nlkleve


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