https://www.inaturalist.org/observations/365959744
The point of this observation is a flag I placed on the property line, but when I look at the map in map mode, it looks like the point is 1.5 m northish of the property line. The map shows the property line very close to the house corner, but in fact this corner (of a covered porch) is about 3 m from the line. (The flowers are a short distance from the flag.)
In satellite mode, I can see the two trees which are on the line. The west one was alive when I surveyed the lot, but is now a tall stump; the east one is in the circle. It looks like the center of the circle is on the line through the trees, which it is.
How can I get this corrected? Back in 2020 when I surveyed it, I took the map of the survey and a list of state plane coordinates to the GIS guy. The plat has an angle more than a degree off and some distances seriously wrong; he called it a cartoon. I don’t know if the map iNat uses is sourced from the county GIS or what.
1.5 m error is within the usual errors made by the different anges of view from the photographing planes and sattelites and their terrain-height corrections.
The satellite images are not going to accurately represent the actual property line as each one is taken at a different angle and because of this points on the ground will shift by meters to tens of meters in different satellite images. Even after orthorectifying.
It’ll let you know where the line is in relation to something else in that same image, but not when compared to a point dropped based on independent GPS readings.
And even without that, on public access maps like iNat, Open Street Maps, Google Maps, Bing Maps, etc, etc use 1.5 meters is well smaller than their accuracy range.
The satellite image shows the trees, not the property line. The trees happen to be on the property line, but the line is defined by a square rod and a pipe, not the trees.
The map in map mode shows the property line, but it’s wrong. That’s what I want to fix.
I have observed twotrees that define a property line.
Even plat maps issued by a county are not precise to the level a survey is, and any online maps that show property lines are probably pulling the info from plat maps. I don’t think you can get the map itself changed, or that doing so is necessary.
And the observation just looks to be a meter or so off. The property corner is at (35.3271826, -81.8718677) according to LandGlide, and the observation is dropped at (35.32714, -81.87189), which are several meters apart.
The square rod, which is the rincĂłn of MarĂa González and the esquina of Dan Shields, is at:
348550.51391,178733.66836 state plane
35°19′37.8707″N 81°52′18.7420″W lat-long DMS
35.327186,-81.871873 lat-long decimal.
The pipe is, as far as I can tell looking at that map, on the north side of Delta Street as drawn, but it’s just south or southwest of the stump, not southeast as the map shows.
MarĂa González is Dominican, if I remember right, and has some relatives living there, but Javier, who asked me to flag the lines, is Salvadoran and just lives there. In 2020 we were all attending the same church, but I switched churches during the pandemic.
Spanish has two words for corner: rincón y esquina. The corner of a house is la esquina if you’re outside the house and el rincón if you’re in the room, but I get confused about which is which when talking about a map of a lot like this.
Treating the earth as a sphere, instead of a 1/300 flattened ellipsoid, affects only the latitude. The longitude is also in error.
It is possible that the error comes from an error in the deed. The deed didn’t close, and I had to get a map drawn by a deceased surveyor from another surveyor to figure out what happened. Parts of two line segments were omitted by whoever drew up the deed.
Where does the property line app get its data from? If the source is Rutherford County GIS, then I have to go back there with the coordinates so that they can fix them.
This observation has a positional accuracy of three meters. Plus, smart phone GPS coordinates also have accuracy issues.
GPS satellites broadcast their signals in space with a certain accuracy, but what you receive depends on additional factors, including satellite geometry, signal blockage, atmospheric conditions, and receiver design features/quality. For example, GPS-enabled smartphones are typically accurate to within a 4.9 m (16 ft.) radius under open sky (view source at ION.org). However, their accuracy worsens near buildings, bridges, and trees.
I put the flag within about 2 cm, having calculated the point 1/3 from the square rod to the pipe, using a survey-grade GPS receiver, which you can see here. The plant is at least a meter from the flag. This is how I usually locate observations while surveying: I measure a traverse point or lot corner in the field, or compute a point and then flag or stake it, then observe some plant or animal near it.
All the ellipsoids used in geodetics have a perfect circle for the equator. Any imperfections are handled in the geoid.
Where does Google get the map data? Does the property line app get them from the same source?
Maybe one of these weeks when I’m not busy with surveys I’ll go back to the county GIS office and see if they have wrong coordinates for this corner.
