Am rather concerned re map accuracy.

Flew from N Yorks to Boston today. Part of route was throgh the stub between Cranwell and Coningsby. I particularly noticed a difference of some 0.5 miles between Garmin (Jeppesen) and SkyDemon re the southern stub boundary. According to SD I reached the s Boundary about 0.5 miles before Garmin said so. One has to be wrong. Pains me to say it but suspect Jeppesen is correct.

Other than that 8410/SD worked flawlessly.

Also location of my home field, Husthwaite is wrong in SD. I know this because when you are on the runway, airfield loaction on SD is about 0.3 miles south.

Conversely, all roads are exactly right  .. going by test when just driving about.

Any thoughts Tim?

David

Why would you think the SkyDemon chart is wrong? Having just checked the AIP description of that zone, and its accompanying chart myself, it looks to be the same as the SkyDemon data. Remember that our data is updated a lot more frequently than Jeppesen so it's possible they haven't taken into account a change of orientation of the stub (just guessing).

The location of Husthwaite airfield is taken directly from the UK AIP, which published information about microlighting sites. If they have got it wrong, please inform them.

Tim

Thanks for reply.

My answers to your Qs are as follows:

1. Why suspect SD? Well Garmin/Jeppesen has been around for some while. If they were constantly getting airpsace busts due to inaccurate maps/gps fixes, then I think it would have been addressed by now. Checked with Coningsby ATC. There have been no changes to boundary in living memory. My Garmin was last updated in july 09 (am ashamed to say!). Manual checking of the s boundary lat / longs N of Heckington does suggest your boundary is further north than MM, Navbox or PFMS. For a MATZ this does not actually matter but for other airspace it does  .. even 1/4 or 1/2 mile which is my estimate of the error. I was not meaning to jump to a conclusion re SD. Also, I do not know where your zone data comes from or how it can be checked. Just seems it may be worth looking at.

2. Have spoken to appropriate people in UK AIP.  There is a discrepancy bewteen their published data and the CAA chart (which shows Husthwaite in right place). Will get this fixed.

Just trying to provide useful feedback so that we all make most of your excellent software.

D

MATZ is the only type of UK published airspace that does not have explicit coordinates describing its lateral limits. I would not be surprised to learn that there are minute differences in interpretation between different data providers, but I can verify that the data you see in SkyDemon for that stub is correct as per the AIP.

You can actually verify this yourself. When creating user waypoints SkyDemon allows you to define them with reference to existing waypoints. Create a user waypoint 7nm out on a bearing of 083 from Cranwell aerodrome and call it StubMiddle. Then create a user waypoint 2nm out from StubMiddle on a bearing of 173 and that user waypoint should lie pretty much (within 0.1nm) on the stub boundary.

I would be concerned if there were even a 1/4 mile error in any SkyDemon airspace data.

Tim

Thanks for this. As you see this has caught my imagination. Have done some checks as you suggest and I realise it does not matter much anyway as MATZ boundaries are not legal for us and you explained they do not have proper co-ordinates.

Doing some checks, I conclude as follows:

1. Proper CTA boundaries are in right place! Checked this via road intersections and co-ordinate cross check with MM, 1/2 mill chart. As I and you expect.
2. But while the 7mile/83 & 2NM 174 test does indeed give a leg length of 2.0 miles on your map, distance to the boundary on Flitestar (Jeppesen) is some 2.2NM. On MM its a bit over 2NM but say 2.1.
3. However, having got me interested, I then looked at the width of your MATZ stubs. On my screen at least, I wd say yours are almost all around 3.8NM. Difficult to make many = 4NM.

But in the scheme of things very trivial I accept. Just need to be aware that there may be differences re MATZ boundaries when exporting routes to Garmin.

D

I've asked our data guy to see whether there's a reason why we may have a 0.1nm discrepancy at either side and perhaps it'll turn out to be something obvious, but I'm not too worried at this stage when that's all it is.

Your comparison to road junctions is interesting. Although our aeronautical data is all accurate, our geographical scenery, which includes roads and coastline, is public domain data which can be off in some places by up to a mile. We deem this acceptable in SkyDemon as it's for aeronautical navigation, but do be aware of the possibility.

Thanks for speedy response as ever.

Must have chosen some accurate intersections!

Re MATZ, agree it does not matter much at all. Just interesting.

D