GTOPO30 Scenery...

Sorry to post here again fellow FS2002 users, but I have another question regarding my scenery I am developing for the UK for X-Plane. The reason I ask here is that there is a fountain of knowledge and also that I have used the same data sources as those used in FS2002 scenery :-I have now got my software to create X-Plane scenery from the Global Land Characterization data and GTOPO30 DEMs. All is well except that some (most?) airports in X-Plane now appear to be slightly raised on small hills above the surrounding terrain. I suppose I could adjust the airfield altitudes but I would like to know why the discrepancy in the airfield data and the terrain data. As I have said, the elevations of the terrain come from GTOPO30 DEMs and the airfield elevations come from DAFIF cycles.I have seen this happen in FS2002 when using third party mesh scenery (albeit about a year ago since the last time I used FS2002 - shame on me, hence this is another reason for posting this question here!).Does anyone have a explaination as to the difference in the data sets please ?RegardsChris Ehttp://www.circle-software.co.ukhttp://website.lineone.net/~flightsimukAvoid AGP texturing = Better Performance ;)