Align background image in 3D editor
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This tutorial describes how you can get a background image properly aligned in your 3D editor, like GMax. By having it aligned properly you can use it to trace aprons and taxiways or to position your objects correctly. So let's get started!
Getting the image
The first step obviously is to get your background image. I can not tell you where to get them for your airport or country, there are just too many differences per county. But I can tell you what kind of background image you need.
Most import is that you get an image in geographical coordinates (latitude and longitude WGS84). This is the same requirement as for images used in resample to make photorealistic terrain scenery. So if you image is in another projection, you first need to reproject it. Most GIS applications can do this for you. Look at this tutorial if you want to use FWTools for it.
Another constraint is that you don't want a very high resolution image. If you have a 10000x10000 pixel image your 3D editor will most like not be able to work efficiently with it. So a slightly lower resolution is not that bad.
The image on the right shows an example background image for the airport Drachten in the Netherlands. You can see there are some back edges on the image, that is because I had to reproject it from the local Dutch projection to WGS84. I will use this image in the rest of the tutorial.
Calculating position
In your 3D editor you are working in flat earth coordinates, so to be able to position the image correctly we need to convert the know latitude and longitude of the borders of the image to XY coordinates in flat earth. The forumulas to do this conversion are well known. So you can do the math with a simple calculator or with Excel. In this case I will be using the ModelConverterX tool since it has a nice function build in for this.
So the first step is to know the latitude and longitude of your image. Maybe this information was provided together with the image or if it is a GeoTIFF image you can extract it using FWTools.
For the image of Drachten the coordinates are:
| Top latitude: | N53.1254080982383 |
| Centre latitude: | N53.1180834291591 |
| Bottom latitude: | N53.1107546132927 |
| Left longitude: | E6.10575772128518 |
| Centre longitude: | E6.12804110910755 |
| Right longitude: | E6.15031695315712 |
As you see I did not only get the latitude and longitude of the edges, but also of the centre of the image. That's because I will be using the centre as my reference point and origin in the 3D editor.
So now the next step is to calculate the XY coordinates of the top-left, top-right, bottom-left and bottom-right corner of the image. Like I mentioned before there are formulas to do so, but I will be used the Coordinate Converter tool within ModelConverterX (see image to the right). In this tool you enter your reference point latitude and longitude, so in our case the centre of the image. And then the latitude and longitude of a second location. As a result you get the XY coordinates. So we need to repeat that for every corner. The resulting coordinates then are:
| X | Y | |
| Top left: | -1488.761 | 813.995 |
| Top right: | 1488.257 | 813.995 |
| Bottom left: | -1488.761 | -814.445 |
| Bottom right: | 1488.257 | -814.455 |
