The only method I'm aware of is to use simconnect to create an AI object within FSX as the user plane flies around the landscape, move it to the lat/long you are interested in (typically relative to the position of the user aircraft), and read the AI object's ground elevation.
This technique is used in sim_probe (simconnect Visual C++)
http://carrier.csi.cam.ac.uk/forsterlewis/soaring/sim/fsx/dev/sim_probe/ which actually places four probes on the landscape. The *source* of sim_probe is included in the package. The technical paper has a section describing the technique in more detail. The bulk of sim_probe is concerned with processing the elevation data it gets back, but hopefully you will be able to work out the basic message loop that picks up the elevations from the probes.
With 4 probes I have not been able to detect any framerate impact on FSX at all, but your requirement could need more samples and I have no idea when the processing would begin to impact FSX. The noob thing to do would be to sample the elevations every frame, but sim_probe uses a slower cycle - once per second I think - which is more than adequate for the application and reduces processing by a factor of 20 or more. It would also be a very inefficient approach to re-probe the same landscape many times within range of the user aircraft - you only need to probe the forward edges of the region in the direction of travel of the user aircraft, and update your internal data structure accordingly.
To sample the terrain in multiple places you can either create one AI probe and move it around during the sample cycle, or you can create many probes, move them all, and sample the ground elevation of each (sim_probe does the latter, and the former is really a simplification of that).
This work is *not* for the faint-hearted. You have to learn C++, the simconnect API, work our how to set up Visual Studio to compile and link the code, learn how to create and manipulate AI objects in simconnect particularly suspending the FSX control that would normally happen, and not least program for the situation where FSX 'culls' your AI object as a normal part of its optimization. You will almost certainly need a set of routines that calculate offsets and distances given a lat/long starting point and maybe a bearing. sim_probe does these things, including a 'heartbeat' that re-creates the AI objects when they've dropped off the map.
By the way if you want to include this data in a *gauge* you will need to create an executable that is *both* a simconnect client and a gauge dll. This is doable but if it's your first time you'd need to create a c++ gauge (or vb?) and get that working (using the panels/gauges SDK), create a c++ (or vb?) simconnect client and get that working (referring to the simconnect SDK), and then merge the two sets of code into a common source file. Even though you know VB, my advice would be to use C++, and *don't* used 'managed code'. Then you'll be able to take advantage of all the samples in the SDK's, and you won't be trying to get users to install the right version of .NET just to get your dopey gauge to run.
The good news is once you get the basic framework working, you can play around with how you process the elevation data that comes streaming back from around the aircraft, and that's actually quite entertaining.
Good luck,
B21
