The projection hell

The different projection systems used in geographical data might be one of the aspects of MSFS scenery design that most designers know hardly anything about. But it surely a very important thing.

Almost each country has its own projection system and although most of these systems will give you the position in either degrees or meters, that does not mean that it will all fit together. So when using data to design your scenery it is a good idea to take a look at which projection system they use.

In the terrain SDK it is written that the MSFS mesh terrain uses the WGS84 projection system. But an interesting question remains which projection is used for the scenery objects (so the coordinates in meters you define in the MDL object file). As objects are in general small, the influence of the projection system will be a small, but it would still be interesting to know.

While working on my Dutch sceneries I first met all the trouble that different projections can give. We got some data that used the Dutch Rijksdriehoekscoordinaten system (the position is defined in meters from a church tower near the center of the country). But compared to the WGS84 projection system, the Dutch RD system also has an additional rotation of a few degrees. So when you think you can just use the position in meters, you end up with your scenery in the wrong location. In the end we luckily found a set of good formulas to do all the transformations to WGS84 and back.

Another example where you will meet different projection systems is when you use a background image in a program like Ground2k4 or SceneGenX. The mesh scenery and the XML style scenery are all defined in WGS84, but a lot of maps are in meters. So in that case you would have to reproject the image to get a perfect match. SBuilder is a tool that has added this reprojection functionality. If you do not take notice of this, you might be positioning your scenery at the wrong position, especially when you get further from the reference point.

For those that have become really enthousiastic about projection systems after reading this, take a look at the Proj4 library. With this library you can easily convert between almost any projection system. At work I have also used this library to build some conversion tools.

I’ve got my own blog

As you can see I do now also have my own blog. Welcome to all who have found it. Most will probably now me from my scenery design activities for Microsoft Flight Simulator. And as the name of my blog suggests I will post mainly about this subject as well. But that’s not all.

Let me start with a slightly longer introduction about myself. Besides MS Flight Simulator and especially the scenery design, I also have a big interest in aviation in general. That all started when my parents took me in a plane for the first time to go on vacation. I was 8 years old at the moment and since then I have been crazy of aircraft.

When I had to decide what I was going to study, the choice was not that hard to make. I was going to do Aerospace Engineering. During the study my interest in simulation grew stronger and in the end I graduated on the “force feedback” of the control column (which was a powerful hydraulic function in that simulator).

Shortly after my graduation, now almost a year ago, I started to work at the National Aerospace Laboratory (NLR), which is the Dutch aviation research laboratory. I do work at the simulation department and part of my job become to design the visual databases (that is how scenery is called in that world) for those simulators. Slightly to my surprise I found out that there are quite a few similarities the MSFS scenery and these visual databases. I will try to post some interesting things about this link between the MSFS “hobby” world and the “professional” flight simulation world on this blog as well.

That was about it for my first blog entry. I hope to see you back later, when I post most useful material.