Sign in to follow this  
Guest cornclose

Scenery Projection Question...

Recommended Posts

Hello there,Although I'm no newbie to scenery development I have never attempted an entire region before. I have a question regarding some Land Characterization Data that I have for Europe. The data is in the 'Lambert Azimuthal Equal Area Projection' format. Basically each pixel represents 1000m x 1000m of land cover information.When I view the data as a raster image, it appears to be slightly different from that which I have seen before, sort of 'skewed' a little. The data I have includes a few bits of information such as Lat/Long of origin and radius of sphere etc.My question is this :-The data seems not to be suitable for direct use for scenery generation, so what needs to be done to it to make it suitable ?I am not 100% sure of terminology but I have seen reference to WGS84 (which I know is a geometric standard) and I believe I need the data to be 'Ortho-rectified' but I don't know what math to apply (I have searched) to make the data suitable. If there are any freeware (or even shareware) programs that do the job I would like to know about them.Otherwise, if anyone can point me in the right direction to get my answers I would be grateful. I am a programmer also so if anyone has technical knowledge of what I might need to do then please don't hold back.Thanks Chaps...Chris Ehttp://www.circle-software.co.ukhttp://website.lineone.net/~flightsimukAvoid AGP texturing = Better Performance ;)

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Hi Chris,I'm not an expert on mapping, but I've had some technical training in the area, and have lately been dealing with some similar issues...Besides knowing the projection of your data, you also need to know the datum. The datum is probably listed on your map or with your data. WGS84 (aka NAD83) is a datum, which is a model of the 3D surface of the earth. A projection is a system of mapping the earth to a 2D surface. What you want to do is to "unproject" your data so you can fit it to the 3D model in FS2000/2002, which is closely modeled on the WGS84 datum. Unfortunately, I haven't seen any freeware/shareware programs to handle that specific task.(But I've been looking at conversion from one datum to another, so maybe its out there). Most GIS mapping packages are quite expensive (thousands of $). The cheapest I have seen is Manifold, which currently retails for US$245. (check manifold.net)If you want some more background on datums and projections, I have a small webpage that has a basic description and some good links:http://chem.queens.edu/maps/While searching for formulae, its good to know what kind of accuracy you require. Surveying requires accuracy down to a few cm, but for flight simming, many kinds of data (such as land class)will still be great if it is accurate to within 50m.good luck!andy

Share this post


Link to post
Share on other sites

Hi Chris.Andy is right on the money, as far as the datum is concerned.WGS84 is the datum used by FS2002. This reflects the actual position of the center of the earth, and the spherical shape upon which the surface is rendered...but that rendering is 3D.This is not the projection. By projection, we mean how a 3D sphere is projected onto a flat 2D surface ( a map ).FS2002 uses 'Platte Carree' or 'Equirectangular' projection. It is also called 'Simple Cylinder' or 'Rectangular', with a standard parallel as the Equator. Each line of latitude and longitude forms a perfectly square grid with 90* corners. A bearing of 45* remains 45* throughout the whole map. The surface is quite distorted... and that's OK, as FS puts is back together as a sphere.Gotopo30 dems are described as 1km data... but the projection is actually a 30 arcsecond grid... Equirectangular.I don't know of any freeware program to reproject the landclass raster data to to Platte Carree... maybe GRASS, but I've never run that program as it sounds incredibly difficult to setup.Christian Stock may know a bit more about this, and any freeware programs that may be used to convert such a large raster image.( I've also heard good things about Manifold, and I am considering getting it. I know Justin Tyme uses it with his mesh... and perhaps landclass data. )If you have world landclass data using the "Global Ecosystem Legend", you may end up shocked to find your landclass data is actually no better than the FS default... as that is what Microsoft uses ( with even the same names! ) Other data legends will have to be converted to the "Global Ecosystem Legend" so that you can use them.=======================TMFViewer shows the "square-gridded" nature of Flight Simulator's data. Christian Stock noted that it would be possible to take screenshots of the worldlc.bgl, and 'stitch' them together to make a world landclass sourcefile. I used Goode homolosine projection to make a world landclass source... and it worked pretty good without conversion... but I'll bet it's "off" a bit. :)I've been using screenshots of TMFviewer for work with Christian Fumey's Ground2k. His program wants an "equirectangular" ( square-gridded ) image... with known NSEW latitudes & longitudes. I load the worldlc.bgl first.. then bgls for VTP lines or LWM polys displayed over that BGL... gives nice, squared geo-referencd screenshots.Dick

Share this post


Link to post
Share on other sites

Thanks Andy for your reply and the information you gave me. It help, because half the problem when researching a subject is using the correct terminology, which once I did, I found the information I was looking for.Thanks Dick too for that additional information, it just further confirms that what I have discovered is correct.Now, I have a little confession to make. The scenery I am developing is not for FS2002, but for X-Plane, but since there is no X-Plane forum for scenery development (well, none that would give answers to this level) and there are many more FS scenery developers than X-Plane ones, I thought I'd ask my questions here. Sorry! (I am also an FS98-2002 user too though!). As for your point Dick about FS2002 using the land class data I am trying to use, well yes I'm sure you're right, but X-Plane certainly does not, and is in need of improvement. I actually intend to use the 'Seasonal Land Cover' data as opposed to the Global Ecosystems as the former has over 250 classes instead of the Global Ecosystems being less than 100.So, this is where I am now :-The data I have from http://edcdaac.usgs.gov (Global Land Characterization) uses the WGS84 datum and was projected in 'Azimuthal Equal Area Projection'.What I need for X-Plane appears to be the same as that required for FS2002. ie. Equirectangular.So I now have a demo version of a program called Global Mapper. See http://www.globalmapper.com for details.Basically the demo version has some features disabled and only allows you to work with one data set at a time, but other than that, it appears to be quite good. One thing that it does allow you to do is to reproject data from one projection method to another - in my case from Azimuthal Equal Area to Equirectangular, which it seems to do with aplomb. Even the full registered version of this is only $124 so I might indulge myself if the demo gets annoying.The only issue I came across was getting the data into Global Mapper in the first place. It allows import of loads of different formats, but it did not appear to allow the opening of simple raster data, in my case 13000 x 13000 pixels with no header information. So I simply opend this raster data in PaintShop Pro 7 and saved it as a greyscale .png file.Then when I opened the png file in Global Mapper it asked for an associated world file, which I did not have, but was able to create from the information I had from the readme of the Global Land Characterization data. Again internet searching helped me to get the data in this world file in the correct format.After that, it was plain sailing, and I now have a data set in the correct format for use with X-Plane. All I have to do now is to finish my program which analyzes this reprojected raster image and compiles the X-Plane scenery files. I am almost there! I think that I will also look into the DEM data for the same region aswell so that I don't just rely on the default X-Plane elevation data. Again, I already have my program reading and writing elevation data, I just need an accurate projection of it.Thanks guys for your input, it helped me look in the right directions.Hopefully although you now know this is an X-Plane project you might still answer any queries I may have in future. That is unless the moderators don't kick me out!Thanks againRegardsChris Ehttp://www.circle-software.co.ukhttp://website.lineone.net/~flightsimukAvoid AGP texturing = Better Performance ;)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this