Up

Answer: The FCC doesn't actually HAVE a standard to determine how accurate your location and elevation data has to be in order to "stay out of trouble".  But here is an answer anyway.

On a non-registered tower:

Short answer: 

Horizontal Accuracy (Lat/Long Coordinates):

Using a topographic map, you should be within 49 feet of your actual location.
Using a GPS receiver, you should be within 39 feet.
 

Vertical Accuracy (Elevation):

Using a topographic map, your accuracy should be within ± ½ the contour interval of the map you are using.
Unless you know exactly what you are doing, do not use a GPS receiver to determine your elevation!

Long answer:

Horizontal Accuracy (Lat/Long Coordinates)

On the FCC instructions for filling out Schedule D of the Form 601 Application for a new license:

Items 7 and 8 For fixed locations, if you entered ‘N/A’; in Item 6, you must enter the geographic coordinates (latitude and longitude) of the location. If you provided an Antenna Structure Registration Number in Item 6, then these items are optional (see Important Antenna Structure Registration Information on page 2 of these instructions). For all other locations enter the geographic coordinates as specified in the table in the instructions for Item 4: 

                If ‘P’ was entered in Item 4, enter the geographic coordinates of the center point;

                If ‘R’ was entered in Item 4, enter the geographic coordinates of the southeast corner of the rectangular area or box. (See also Items 21 and 22 for the Northwest corner)

 Enter the latitude using the format DD-MM-SS, where the degrees (DD) term can have a value in the range of 0 to 71, minutes (MM) can range from 0 to 59, and seconds (SS) can range from 0 to 59.9. If desired, seconds may be rounded to the nearest tenth of a second (in which case, use the format DD-MM-SS.S). In the right corner, specify the direction as either N for North or S for South.

 Enter the longitude using the format DDD-MM-SS, where the degrees (DDD) term can have a value in the range of 64 to 180, minutes (MM) can range from 0 to 59, and seconds (SS) can range from 0 to 59.9. If desired, seconds may be rounded to the nearest tenth of a second (in which case, use the format DDD-MM-SS.S). In the right corner, specify the direction as either E for East or W for West.

 All coordinates must be referenced to the North American Datum of 1983 (NAD83). This information can be determined in many ways, including a GPS receiver, a 7.5-minute topographical quadrangle map of the area, or you may consult the city or county/borough/parish surveyor in your area. Topographical maps may be purchased from the U.S. Geological Survey, Washington, DC 20242 or from its office in Denver, Colorado

That doesn't tell you a lot without knowing what the FCC expects for accuracy if you use a GPS receiver, or a 7.5 minute (commonly called a 1:24,000 scale) map.

According to an footnote to a Public Notice released in 1988 ( DA 88-316 -- 3 FCC Rcd 1478 FCC website)  (local copy):

Site coordinates derived from 7.5 minute (1:24,000) USGS paper maps by spotting the tower / antenna site on the map, and then interpolating the latitude, longitude, and elevation data from the map, have a horizontal accuracy of no better than ± 12 meters and a vertical accuracy of ± ½ the contour interval. In the continental U.S., one second of latitude or longitude is about 30 meters. Assuming that an applicant can accurately spot the proposed tower / antenna site on a map, decimal seconds cannot be accurately interpolated from the map.

A horizontal accuracy of ± 12 meters means you should get within 39 feet of your actual location.  A low-cost consumer grade GPS receiver should get you within ± 15 meters (49 feet), although  many will get you closer.  A WAAS corrected GPS receiver should get you within 3 meters or 10 feet, assuming you have the correct parameters setup in your GPS receiver, and that you take the most accurate reading your receiver will allow you.

So, if you are using a topographic map, your accuracy should be within 39 feet of your true location, and ± ½ the contour interval of the map you are using. 

Even if you are using an inexpensive GPS receiver, your coordinates should be within 49 feet of your true location.

Vertical Accuracy (Elevation)

The vertical, or elevation accuracy for a topo map is expected to be ± ½ the contour interval.  The contour interval is stated on each 1:24,000 topo map, and in most of the US is either 10 or 20 feet.  In mountainous regions it can be 100 feet between contour lines.  If you are using a topo map with 20 feet intervals for contours, then you margin of error for elevation should be no more than 10 feet.  Of course, there is a margin of error in 1:24,000 topo maps, and its margin of error is also ± ½ the contour interval, so you could actually be off as much as another 10 feet from the actual elevation, although not from the map elevation. 

A GPS receiver is less accurate for elevation data than it is for coordinate data, usually 2 to 5 times as inaccurate due to several factors (If you really want to know, read this paper published by Auburn University or this article by NOAA).  Assuming a worse case that your GPS elevation data is 5 times as bad as your coordinate data, then your elevation could be as much as 200 feet off!  There are ways and methods to vastly improve this figure, but you have to know what you are doing, and what options to choose when you purchase a GPS receiver.  Because of this, unless you do know what you are doing, I do not recommend an elevation reading from a GPS receiver for licensing purposes.  Generally, if you can provide me with accurate GPS coordinates, I can use a USGS 3" terrain database to get an elevation that is generally acceptable to the FCC.

On a registered tower:

If the tower you are putting your system on has been cleared by the FAA due to height above 200 feet, or closeness to an airport, then that tower must be registered with the FCC.  When it is registered with the FCC, then all the technical data (latitude, longitude, tower height, ground elevation, etc) for that location are on file in the FCC's ASR (Antenna Structure Registration) database.

If you are putting your system on an FCC registered tower, then your coordinate data for your radio system MUST match the data for the tower.  On page 2 of the Form 601 instructions, the FCC says:

IMPORTANT ANTENNA STRUCTURE REGISTRATION INFORMATION: The Commission will evaluate your application for a fixed location based on the Antenna Structure Registration number you provide (if required). If you mistype your Antenna Structure Registration Number, or provide location data (latitude, longitude, elevation, and structure heights) that is inconsistent with the data found in the Commission’s Antenna Structure Registration database, your application will be dismissed.

Inconsistent data is defined as follows:

                1. Latitude and Longitude data provided on Schedule D differs from the data in the Antenna Structure Registration database by  more than 1 second; or

                2. The height or elevation reported on Schedule D is more than 0.5 meters (1.5. feet) greater than the data in the Antenna Structure Registration Database; or elevation is more than 3 meters (9.8 feet) less than the elevation listed in the Antenna Structure Registration Database.

So, if your license application is more than 1 second different or 1.5 feet different than the data that the FCC has for the tower you are placing your system on, they will NOT accept or act on your application.  It will be returned (if the coordinator even lets it get to the FCC).

As long as you can provide the ASR for the tower you are going on, finding all the coordinate data is as easy as querying the FCC's ASR database, As a matter of fact, we do this as a matter of course for all new applications.  The only thing we really need to know beyond that is the height of your antenna on the tower.