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I'm studying for the commercial drone certificate. In Tucson, AZ the class C airspace covers about half of the city, but the charts don't list the exact point that it ends. Here's a portion of the VFR chart:

Tucson airspace

How exact are these airspace borders? Could one city block north or south get me in trouble if I don't check in with ATC?

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    $\begingroup$ Welcome Mitch! Areas are defined using their geographical coordinates. So it's exact. What is the airspace you are asking about? $\endgroup$ – mins Sep 2 '16 at 17:54
  • $\begingroup$ davis monthan afb class c $\endgroup$ – Mitch Sep 2 '16 at 18:07
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    $\begingroup$ When in doubt or close, call Tower and state your intentions. They'll approve you, and if you cross into their airspace you will be legal. If you stay outside their airspace, you asked approval for no reason, but no harm done. $\endgroup$ – abelenky Sep 2 '16 at 19:48
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The boundaries are defined more precisely than the chart can show. What you see on the chart is a visual representation of the text published in JO 7400.9Z (or the most recent version of it):

AWP AZ C Tucson, Davis-Monthan AFB, AZ
Tucson, Davis-Monthan AFB, AZ (lat. 32°09'59"N., long. 110°52'59"W.)
Tucson International Airport, AZ (lat. 32°06'58"N., long. 110°56'28"W.)

That airspace extending upward from the surface to and including 6,600 feet MSL within a 5-mile radius of the Davis-Monthan AFB to the points where the 5-mile arc joins a 5-mile arc from the Tucson International Airport, AZ, Class C airspace area, and that airspace extending upward from 4,200 feet MSL to 6,600 feet MSL within a 10-mile radius of Davis-Monthan AFB to the points where the 10-mile arc joins a 10-mile arc from the Tucson International Airport Class C airspace area.

So in theory, if you're at 5 miles and 1 foot from that lat/long point then you're outside the class C's inner area. But the chart has to show that information in a way that's usable for practical purposes, which means that there has to be a visible line on the chart and that line has thickness. It looks to me like the airspace boundary lines on the chart are roughly 0.25nm or about 1500ft thick, and I have no idea where the boundary really lies on it. You simply have to consider that as a 'margin of error' in this case and plan accordingly. If in doubt, definitely contact ATC.

As for getting in trouble, 1500ft is probably too close for comfort for a manned aircraft, and ATC reacts on what they see on their radar, not what you see on your GPS. It's an interesting aspect of this question that you can probably be more precise with a UAV than a manned aircraft, and 1500ft might seem like more than enough buffer for you. But again, if you have any doubts or questions I would ask ATC directly.

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    $\begingroup$ Here is a map based on the above description which shows the inner areas on a map that you can have satellite view on and zoom in to street level (note that in this JO, "miles" refers to NM). (I'm assuming that the outer areas won't be needed since the drone will have to fly below the altitude of the shelf.) The accuracy is limited to that of the map itself. $\endgroup$ – Lnafziger Sep 3 '16 at 5:17
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    $\begingroup$ Also, that JO is accurate as of July 28, 2015. If you need current data, read the JO and it has a link to changes since that time. $\endgroup$ – Lnafziger Sep 3 '16 at 5:24
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The airspace is defined by a written publication. Most likely, for this Davis Monthan Class C airspace the definition is based on the 5 and 10 NMi rings from the published, and surveyed, airport reference point (this could be the control tower location, for example) on the airport master record. For example, you can find one point here: DMA Airport Master Record There is survey accuracy data, but I don't think that is really the point of your question. The answer is that as far as the FAA is concerned that 5 NMi ring is THE airspace definition and your UAV is either in it or not. There isn't a "grey area" of "kind of in it.". So, one side of the block is 4.95 NMi, and is in the Class-C, while the other side, at 5.02 is outside.

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