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.
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?