# Measuring distance on Google Maps: improved version **URL:** \ **Category:** Miscellaneous and Personal Stuff I Must Share\ **Created:** [October 17, 2025, 5:47pm UTC](https://boards.straightdope.com/t/measuring-distance-on-google-maps-improved-version/1024056 "2025-10-17T17:47:49Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Timz](https://avatars.discourse-cdn.com/v4/letter/t/6a8cbe/32.png) [@Timz](https://boards.straightdope.com/u/Timz)\ **Post date:** [October 17, 2025, 5:47pm UTC](https://boards.straightdope.com/t/measuring-distance-on-google-maps-improved-version/1024056/1 "2025-10-17T17:47:49Z")
[I wrote this a couple years ago] This post is about investigating Google Maps’ accuracy – specifically, how accurately you can measure distances off of it. If you’re not interested in that, no need to read any further. Google will give you a straight-line distance between any two points on the map, but it turns out you can get a much more accurate distance than the one Google gives you. (Some? Most? of the time, anyway.) A few feet east of the red marker [https://goo.gl/maps/dS1v59YwGTLDhX797](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2FdS1v59YwGTLDhX797) you see the shadows of two posts – they’re four-inch steel pipes. Two more posts just east of the marker at [https://goo.gl/maps/HQgWaFZLFsJo6Q3bA](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2FHQgWaFZLFsJo6Q3bA) Google says the southeast post of the west pair is 3275.4 feet from the southeast post of the east pair. Actual distance is 3281.1 feet, correct within 0.1 foot. A 0.2% error is typical for a Google distance; the problem is that when Google calculates the distance it assumes a spherical Earth, so in an extreme case (a north-south line at the equator, or at a pole) it’s off by 0.5%, that being how unspherical Earth is. Fortunately we can hope to eliminate that particular error, because Google gives us the latitude-longitude wherever we click on the map. We take the lat-lons at the ends of the line we’re measuring and calculate the distance ourselves, on the unspherical Earth. Good a guess as any: Google says the posts are at 37,80319465N 122.3174799W and 37.7997694N 122.3069758W. (You right-click on the map and it gives you the lat-lon to five decimal places; left-click on that lat-lon and it stores the lat-lon to 15 places.) On the GRS80/WGS84 spheroid those two lat-lons are 3281.35 ft apart; if we want to be quite correct we’ll cut it down to 3281.33 ft since we’re below the spheroid. So the error is 3 inches – for all we know, just because I can’t expect to get the cursor point in the exact right spot, even at maximum close-up. The lat-lons aren’t correct to 3 inches, of course. They’re probably off by 6 feet. But the error is the same 6 feet, in the same direction, for both posts. Two more pairs of posts [https://goo.gl/maps/cDM2zhggye2hPpSV7](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2FcDM2zhggye2hPpSV7) [https://goo.gl/maps/XiHpkpoHiJ1xAd2D9](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2FXiHpkpoHiJ1xAd2D9) As before, we get the lat-lon of the SE post in each pair. Based on Google’s lat-lons, the calculated distance from the west post to the next one is 492.58 ft; actual distance is 492.74 ft, again correct to better than 0.1 ft. Calculated distance post #2 to post #3 is 918.39 ft; actual is 918.12, Calculated distance post #3 to the east post is 1870.39 ft; actual is 1870.24. Judging by the shadows, the posts likely were photographed together, on one picture – no seams between them, where Google stitched together two pics. We have no idea how much area each pic covers, or how much error is introduced by a seam. But as it happens, we can easily enlarge the investigation. See the white dot at the center of the concrete rectangle, just southwest of the red marker? [https://goo.gl/maps/935UeVSad5b9b7WE8](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2F935UeVSad5b9b7WE8) That’s the concrete around a fairly new survey marker; NGS gives the marker’s NAD83(2011) lat-lon correct to something like a centimeter. So we can see how far off Google is, and how consistent its error is. Turns out it’s more consistent than you’d have thought possible. Google’s lat-lon for the above mark is 6.1 feet from the actual position, in azimuth 290 degrees. Next survey marker is the concrete dot a few feet south of the red marker [https://goo.gl/maps/8NvZi2pQiG9oywQA7](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2F8NvZi2pQiG9oywQA7) There, Google’s lat-lon is 6.28 feet from the actual lat-lon, in azi 288 degrees. Next marker is the dot a few feet SE of the red marker [https://goo.gl/maps/isEiQmDkGb3BDgdG6](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2FisEiQmDkGb3BDgdG6) Error there is 6.46 ft in azi 292 degrees. Next marker is at the center of the fence rectangle just south of the red marker [https://goo.gl/maps/2zGQyYtMZMuorQvv8](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2F2zGQyYtMZMuorQvv8) Error is 5.68 feet, azi 289 degrees. Next marker is next to the red [https://goo.gl/maps/TG6wUVxfnmfbu7dF8](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2FTG6wUVxfnmfbu7dF8) Error 6.20 feet, azi 291 degrees. Last of the new markers is 3-4 ft NE of the red [https://goo.gl/maps/2qp9okLpMM6K1ieL7](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2F2qp9okLpMM6K1ieL7) Error 6.20 feet, azi 293 degrees. There’s another mark at the foot of Clay St, downtown. It’s not visible on Google, but with a little extra work we can learn Google thinks it’s 6.10 ft from its actual, in azi 292 degrees. And one more, also not visible, set in the concrete block at [https://goo.gl/maps/um23MWQC9VVkZiTA8](https://www.trainorders.com/discussion/warning.php?forum_id=1&url=https%3A%2F%2Fgoo.gl%2Fmaps%2Fum23MWQC9VVkZiTA8) Google thinks the north corner of the block is 6.98 ft in azimuth 292.5 degrees from its actual position. Ignorant guess: maybe the error is larger here because this is the only marker that’s not at ground level – it’s a couple feet up in the air, and if the camera’s not vertically above it, Google can’t quite fix the resulting error. Which would mean Google doesn’t do as well when marks are at different elevations, instead of all being near sea level as my marks are. Maybe eventually I’ll get around to enlarging the investigation further, checking survey marks farther away. NGS has numerous NAD83(2011) marks in San Francisco and elsewhere, miles away from the ones I checked. (I ought to do it before an earthquake throws everything off.) I left out some details in the discussion above; if you have questions, I may have answers. Or maybe not. ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 17, 2025, 8:18pm UTC](https://boards.straightdope.com/t/measuring-distance-on-google-maps-improved-version/1024056/2 "2025-10-17T20:18:48Z")
Is it any better/worse in other similar tools, like OpenStreetMap or Google Earth or QGIS or ArcMap? PS Your links are all pointing to a redirector from [trainorders.com](http://trainorders.com) (where apparently your original post was). ---
**Author:** ![Timz](https://avatars.discourse-cdn.com/v4/letter/t/6a8cbe/32.png) [@Timz](https://boards.straightdope.com/u/Timz)\ **Post date:** [October 17, 2025, 9:56pm UTC](https://boards.straightdope.com/t/measuring-distance-on-google-maps-improved-version/1024056/3 "2025-10-17T21:56:17Z")
Do any of them give the correct distance? I assumed none of them was more sophisticated.than Google Maps. (Come to think: I guess you have no way of knowing whether they’re correct or not, aside from checking whether they’re better than Google Maps on the examples above.) ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 17, 2025, 10:01pm UTC](https://boards.straightdope.com/t/measuring-distance-on-google-maps-improved-version/1024056/4 "2025-10-17T22:01:38Z")
I think it depends on your use case and needs. The proper GIS tools have various projection tools, different geoids, tilesets, and georeferencing tools. But none of that will beat ground truthing with surveying equipment or precision GPS units. It really just depends what you’re trying to accomplish and what sort of precision is actually needed for it… (I’m not a GIS professional, just someone who dabbles in it) ---
**Author:** ![Reply](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/reply/32/15952_2.png) [@Reply](https://boards.straightdope.com/u/Reply)\ **Post date:** [October 17, 2025, 10:05pm UTC](https://boards.straightdope.com/t/measuring-distance-on-google-maps-improved-version/1024056/5 "2025-10-17T22:05:35Z")
> [@Timz](#): > > Do any of them give the correct distance? I think it depends on how you define “correct”. The measurements are all relative to something, and that something will change over space and time. Landmasses and defined benchmarks will shift over time just due to tectonics, and generally a local reference point and a regional and recent projection will be better for certain kinds of measurements than more global projections. Measurements for real estate surveying within a county would be quite different than scientific tracking of glacial mass changes around the world, for example. It just depends. (And I don’t know all the details) ---
**Author:** ![Timz](https://avatars.discourse-cdn.com/v4/letter/t/6a8cbe/32.png) [@Timz](https://boards.straightdope.com/u/Timz)\ **Post date:** [October 18, 2025, 5:22pm UTC](https://boards.straightdope.com/t/measuring-distance-on-google-maps-improved-version/1024056/6 "2025-10-18T17:22:31Z")
So no way to find out whether the two posts are 3275 feet apart or 3281 feet apart, or some other distance? For decades, instrument makers have been claiming to be able to measure 1000-meter lines to an accuracy of a centimeter or two. Think that’s hokum?