After several mails and questions on the support forum I decided to wirte this tutorial. Main reason is to stimulate the design of sceneries and to have people publish them. It has been a while since I did my last scenery so I might have forgotten some details or put them in the wrong order. Don't hesitate to mail me : .
In this tutorial I'll use the design of the Inner-Oslofjord scenery as an example.
Until now I have always used 1:50000 maps as a starting point. Satellite data for Norway is not detailed enough. The grid size for free DEMs is 1 km. Some pictures of the area are very useful in order to compare the result with the original. So for the Oslo fjord I used the maritime map which is has 1:50000 scale, European (1979) datum and long/latitude (geographic) projection. However this map doesn't cover enough ground to have a nice hilly background. For that I used a land map (1:50000, WGS84 and UMT). Since the land map is only used as background and the maritime is the one that models the area you sail on, I used the European datum & long/lat projection as parameters in the rest of the process.
I scan the maps normally at around 150 dpi. The image gets then quit large but I prefer to work in high resolution and then later when generating the final DEM resize it(max size for the scenery designer is 2048*2048 (i think). I work in 24 bits colour in order to see a clear map. Later this will be converted to the 8 bit bitmap used by the DEM builder. Use this map as background in a program like Photoshop and add a transparent layer that will be used to copy the contour lines. For each height another color is used. I normally draw lines every 20 meters. A nice thing to use here are color swatches in Photoshop. Give each color the height as name. These colors swatches can then later be used as a palett when converting the image to indexed colours. The colour swatches I use in Photoshop can be found here.
There is a small mistake where I missed 20 meters (find it and correct it;-). An example of the use of the colors can be seen below:
For contour lines be sure to use a pencil of 1 pixel . For the water areas I often use a larger pencil. Big empty areas take a long time to interpolate later in the DEM builder. An extra white fill-layer can be added between the contour lines and the map. This one can then be switched on to see a intermediate result.
After all contour lines a copied we hide all layers except for the contour layer. Now we can reduce the resolution if we have to make pixels (save the full resultion one, might come in handy). Convert the image from RGB to index color using the color swatch file as custom color table for the conversion . You'll end up with something like this:
Now you are ready to import it into the DEM Builder.
A problem with the DEM builder is that bmp files can only be imported as a scene projection whilst our map had lon/lat projection (or geographic). This results in errors that make it difficult to get correct coordinates if we work always in that projection. What we need to do is to import the bmp file as scene but with long/lat coordinates set to 0 degrees. For these coordinates the projections result in the same map/coordinates. Later we will convert the DEM and move it to the right coordinates and spacing. For now use lon/lat 0/0 degrees and spacing can be anything, say 10 meters. Min and max height can be calculated. E.g. for the color map in this example min is -100 meters and max is 5000 meters (a span of 256*20 meters).
What we do first is to set everything above the highest altitude (the black parts) to 'no data' . The pixels will then later be interpolated to form a correct scenery. After that use the map from photoshop as a background map. You'll see the contour lines in various gray-scales on top of the map. What to do now is to fill out the details. That is the areas where the 20 meters drawn in Photoshop are to coarse. For example a island of only 5 meters high or a top of 355 meters. Also some hills have e.g. one contour line at 20 meters for a large surface. In that case it is wise to set some point of say 22 25 and 30 meters. I f don't do that the hills become very flat.
Save the result after filling in the details. This non-interpolated DEM can later be used when you want to correct the scenery, e.g. make a spot deeper. Now it is time to start the interpolation. When working on a large DEM it is wise to do this interpolation in steps. So interpolate smaller parts of the DEM at a time. Sometimes an error in the drawing will result in sharp star formed peaks which are very unrealistic. When you interpolate a small piece of DEM you can easier go back one step and correct without throwing aware a lot of computer time.
Once the whole DEM is ready interpolated set convert the DEM to the correct projection (geographic in this case). After the conversion fill in the right center coordinates and set the spacing correct by picking two point and divide the number of degrees by the number of pixels between them.
Once that is done we must convert the DEM back to the projection accepted by the scenery designer, i.e. scene projection. After the conversion the DEM will be a bit reshaped and some 'no data' areas will exist, interpolate these or crop the DEM. Now we can reduce the resolution if we have to many pixels (save the full resultion one, might come in handy).
Now your DEM is ready to be imported into the scenery designer.
A tutorial on the scenery designer and creating objects etc will follow soon.