I used to think that Google Earth was the "be all and end all" for accuracy in afcad creation. Then, with experience, confusion, consternation and finally some dissatisfaction set in. At this point, it's "resignation".
What I mean is this. You have to realize that freeware satellite imagery is dated to varying extents in different areas and by various sources. The primary/most often used freeware sources are Google Earth, Yahoo Satellite, MS Visual Earth and AskSatellite. These four are readily accessible via the web and Sbuilder/SBuilderX.
As I have begun to use SBuilderX more and more frequently, it's become quite obvious that I have to take each of them with grains of salt for the following reasons.
1. Lack of dating
2. Varying quality
2a. Images are assembled by splicing rectangles together. This is an automated process and errors in matching do occur - as seen when 2 sections of a runway or taxiway don't match up perfectly.
2b. Weather can and does obscure ground features as well as creating inconsistent coloring.
3. Image inconsistencies over an area. Seasonality or seasonal variances are sometimes obvious - check out Google Earth's coverage of Vladivostok.. then slide NW and check out what happens. Another area to check out is Polyarnyy and Severomorsk - green areas become sepia toned - obviously not from the same set of imagery.
4. Apparently deliberately introduced blurriness or manual distortion over an area. This is, I believe in response to government requests/restrictions relating to security concerns especially around airbases and other military facilities. Check out Leeuwarden, Netherlands.
So, where does that leave afcad or scenery creators? Sigh. With a tougher job than it first appears. What I think is the most accurate, for me, is to simultaneously slave MSFSX to AFX and to Google Earth whenever possible using FSEarth software. After drawing my afcad, I go to the same location in SBuilderX and check out the accuracy against Yahoo Satellite, MS Visual Earth and AskSatellite by slweing to various points in the afcad and using the Display Aircraft feature overlaid against the Display Background function. Usually, any disagreements are pretty obvious and can be traced to facilitiy upgrades and "imagery dating". So I tend to go with the majority view - accuracy to within 15 feet works for me 99.5% of the time.
CelticWarrior wrote:Newer AFCAD-type programmes allow adding a background image to help with the design (I'm using ADE btw).
I understand how the images are taken by the satellites and therefore it's entirely possible that some distortion is bound to creep in. But how much? Does distortion occur when the image is saved and applied inside the CAD programme? And how much can we trust the GE image over the airfield which MS has given us?
Hmm. You want specifics? Okay, try this.
The accuracy of using background images in any afcad creation program is related to "calibrating" the image to the afcad design area/window and to image resolution.
Image calibration in AFX, for example, relies on accurately locating the four corners of a rectangle-shaped picture whose orientation is along the 0-180 degree line. The more accurately those four points can be pinpointed and entered into the AFX program, the more accurately the background picture can be drawn.
Edited to add: If you "misplace" corners, the image gets stretched - normally along the north-south or east-west axis.
end edit
That only deals with part of the problem, however.
The second part of the problem with using background imagery to draw/place afcad features relates to the resolution of the picture itself. Like Xerox pictures blown up to huge sizes, inaccuracies and distortions begin to creep in the more the image is blown up in relation to its original size.
So, I can place a picture very accurately and still have a blurry image when I zoom in - something that actually prevents me from determining to a certain degree whether an apron skirt is 5 meters wide or 7.5 meters wide, for example.
I could write more... I go by the axiom, Trust but Verify, and even then I'm usually accepting a certain level of inaccuracy in what I've created.
Ford
Edited to try to correct some spelling/typo errors.