A (probably?) very basic question about spatial error it's about time I knew how to think about and answer correctly.

I have some data plotted from XY coordinates. I look at them and it turns out they appear in places where they shouldn't -- but only by a little bit. E.g. in the image below, the points appearing on land along the sides should be in the water.enter image description here

The errors range from maybe 15 meters to 140 meters ... but usually about 30-50 m or so. How do I know why these are off and if there's any way to fix it? Is it:

1). Because I've incorrectly defined my coordinate system somewhere along the way? (I'm not sure if the original coordinate system of the points is known). But then, wouldn't they be astronomically off target, not by just 40 meters? Plus, it doesn't look like shifting them all in the same direction would fix the issue.

2). Because the resolution of the base layer doesn't capture the shoreline well enough and the points end up looking like they're on land? I've tried it with several sources for the background and none seem to match.

3). Error inherent to the GPS unit used to capture the points? I thought 100+ meters was a little high for anything above recreational-grade.

How do I know what's causing this? Have I done something wrong (#1) or is this sort of thing inevitable sometimes? How do I adjust for it to represent these points correctly, or verbally state an appropriate caveat about the accuracy?

  • I vote Reason 2, look how choppy the shoreline is digitized.
    – ericoneal
    Commented Nov 1, 2013 at 18:53
  • 1
    My first thought is that it's a combination of 2 and 3. Also, it sounds like these coordinates have been field captured. Are you using a data logger or is someone just writing down coordinates (introducing human error to the equation)? Commented Nov 1, 2013 at 18:57
  • I'd be very surprised if anything was written by hand, I imagine a GPS unit was used & coordinates exported from that. But you never know...
    – user14175
    Commented Nov 1, 2013 at 19:06
  • IS the image in the correct location? (how do you know that it is correct?)
    – Mapperz
    Commented Nov 1, 2013 at 19:15
  • Well, I know straight from the source that the points are supposed to appear in Sturgeon Bay / the Sturgeon Bay Ship Canal, which is only like 10 miles long and sometimes as narrow as 125 ft., so they're not WAY off base.
    – user14175
    Commented Nov 1, 2013 at 19:27

1 Answer 1


Any of the possibilities you've identified is possible, and resolving the problem will require detailed analysis.

If the data was collected a long time ago, then Selective Availability could give you the sort of GPS-derived errors you're seeing. Older GPS sets weren't as good, especially the presence of multipath. While unlikely, if all the points were collected on a day with nasty ionospheric conditions (e.g. during impact of a mass-coronal ejection), that could also be a contributor. Investigating the history is probably the only real option here.

Clearly the map isn't really high resolution, and a better map could help. Given that you've already tried that, there could also be a difference in the Coordinate Reference System (CRS) you're using for showing the map vs showing the points (e.g. there are 10s of metres difference between NAD27 and WGS84 or NAD83). You can project them to a common CRS, but you have to make sure that you're projecting from the right CRS. This might take some experimentation with choosing a source CRS, and you run the risk of introducing experimental bias.

  • Hi, thanks for the helpful reply. The points are only 1-2 yrs. old so it's not selective availability. But the projection issue I don't fully understand. I'm not doing analysis and just want to line them up, so do I need to actually find out what coordinate system they were collected in, in spite of on-the-fly projection in ArcGIS? And, why can't I just strip both background & points of any projection information, then define it as whatever I want, as long as they're the same?
    – user14175
    Commented Nov 2, 2013 at 16:37
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    The point is that the items are referenced to something. If its not the same something, its not going to line up.
    – BradHards
    Commented Nov 2, 2013 at 23:17

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