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I'm plotting lines and objects on a map and want to verify scales with real, physical objects on the map. Is there a list of physical features that are known distances?

Perhaps like the runway at Laguardia is 7000 feet long, exactly from this point to this point.

Or, this particular road is exactly due North, and this other is East-West.

Where is the calibrated list of map features bridges, roads, runways or road intersections that have known dimensions?

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    Maybe add which software you're working with? Because normally all data is aligned north, and you can easily measure distances in a GIS.
    – Erik
    Jun 15, 2020 at 14:37
  • I'm working with open source ESRI imagery and also Google Earth. Both have different answers with their measured distances. Both are, in fact wrong to a certain extent, but it's unclear which is less wrong and by how much. I need surveyed landmarks to calibrate Jun 15, 2020 at 15:11
  • No, you think you need landmarks for calibration. But probably it'd be way easier to do the measuring in QGIS, after choosing a fitting CRS.
    – Erik
    Jun 16, 2020 at 6:02
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    Runway lengths are published by the FAA, also reproduced in many pilot-oriented websites.
    – Stu Smith
    Jun 16, 2020 at 6:15
  • runways would be quite useful but where are the starting and stopping locations? I guess it's an accuracy issue. It could work but I think there is better available Jun 20, 2020 at 18:52

2 Answers 2

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You can measure known distances in Google Earth using runway centrelines, touchdown markers or side markings on airfields - these are usually exactly marked based on standards from the local aviation authority.

You want something which is clearly marked, large, and with a long enough distance between or along markers to reduce measurement errors. From the FAA page on "Airport Marking Aids and Signs", you'll see mention of 'precision instrument runway' markers that include touchdown zone and side markings. Pilots and aviation authorities are rather concerned with precise measurements and I have measured side markings in a few countries and found them to be normally obsessively exact, except in some remote dirt airstrips.

Using the QuickMapServices plugin in QGIS, I add the Google Satellite and Bing map layers, confirm that it's using the 'ellipsoidal' option (which uses the default WGS84 ellipsoid EPSG:7030 (visible in Project Properties | General | Ellipsoid)), and check the distances between touchdown zone markings.

For example, Daytona Beach International Airport runway 25R has 4 touchdown markings - should be 1500 feet between them (FAA regulations):

enter image description here

Caveats

  • This relies on a number of assumptions (and I'm fully expecting howls of outrage and hopefully some good advice from others after posting this):
    • The imagery is draped over a good DEM model (good assumption for international airports in USA / Europe). Check Kilimanjaro International Airport for example of some distortions - rw 27 side lines are straight on the ground, not from GE: enter image description here
    • Markings were measured accurately on the ground. I've only found significant variation in small airfields (and a minority of those), and never found more than a 0.1% variation on instrument/precision runways out of the 5 I've checked.

I'm not a member of an obsessive club of runway measurers ... during our aerial wildlife surveys ("Systematic Reconnaissance Flights" - SRF) we measure or place ground markers to calibrate visible "strip widths" as seen by human observers, and I've worked in 9 countries on airfields of various sizes. Actually I take it back, I guess SRF practitioners are obsessive runway measurers.

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This sounds like you would need to survey yourself on the ground or if there is national mapping organisation there use their data.

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  • measuring myself is one possibility. it will be a good exercise, but I don't think I'm the first to need this. surveyed landmarks for calibrating maps is surely something publicly available. but where? Jun 15, 2020 at 15:14
  • Marc, what you are wanting is something that has huge commercial value, you will have to pay for it to be accurate.
    – mh_perth
    Jun 16, 2020 at 11:03
  • indeed, this is quite the valuable commodity. But have you seen the US National Geodetic Survey Data Explorer? Shazam. There is considerable investment in this network. It's all publicly available: geodesy.noaa.gov Jun 20, 2020 at 18:50
  • not the greatest data tbh.... not that impressive.
    – mh_perth
    Jun 22, 2020 at 11:22

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