I have a parcel shapefile for a municipality I'm working with. When I bring the shapefile into ArcMap, I change the fill color to clear and add a basemap. The parcel lines match with the the imagery from the basemap a little, but not 100%. For example, notice the parcel lines for the cul-de-sac in the upper right portion of the picture below don't line up.

The parcels are in the correct state plane coordinate system, although I just added the parcels into ArcMap, projecting on the fly rather than using any projection tool. I don't know if I need to do any formatting to the basemap.

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Many parcel maps were created without the use of aerial ortho photography, if they used photos they were distorted and then they tried to get a best fit. Some were created by reading deeds or survey plans and then this was pieced together, if they were recent they used ortho photography.

The roads are really right of ways(ROW) on tax maps, i.e. may be 50 foot wide and the actual road is 33, usually they were drawn 1/4 inch wide on 100, 200, and 400 foot scale tax maps. So unless they used planimetrics for an edge of pavement they rarely match to the actual road. If the tax map shows a boundary line 100' from the ROW, and you use 100' from the road, you end up placing the parcel in the wrong location.

The cul-de-sac was probably placed by an autoCAD site plan before it was built as they need lots to sell. The end product was the homes showing the misplacement of the CAD drawing, and it was never adjusted. Basically you see a compilation of different materials at different scales with their own errors knitted together by someone using some knowledge and luck. Thus the reason most Tax maps have disclaimers saying for inventory purposes, as this protects the municipality from legal issues as most boundary disputes need to be settled in court.

A side note, some deeds say they own to the middle of the road, these are never drawn on a tax map this way.

  • Yep, the parcel data is not survey quality data. Join the club. Also consider the quality of your imagery. There is going to be distortion in a georeferenced image and less so in a georectified image. – GBG Jun 1 '17 at 15:28

That example is quite awful. I would think it was a rural area that has seen some subdivision recently, but let me explain.

In a previous position one of my tasks was to enter sealed plans into the DCDB (digital cadastral database) using origin benchmarks, bearing and distance. At the end of entering the plan it would not fit with the existing cadastre, usually by a centimetre but up to two metres in rural areas, as that was created from records dating back many years with modern equipment for the time with benchmarks surveyed with what would seem to us as antiquated equipment, so the entered plans were massaged to fit... and not using aerial photography because there is nothing to see on an orthophoto at that time because development cannot even start until the survey is complete and registered.

It is much simpler to massage the few lines as they were entered than to move the whole existing cadastre to match and parcel boundaries must align. While I was there the GIS department undertook a rectification of DCDB to modern benchmarks to resolve that ancient error - it cost over a million dollars! Whilst checking the progress it was noted that many property owners strayed outside the bounding lines as defined, some by quite a bit; in one case a developer doubled the size of a lot by fencing off a large chunk of the adjoining parkland, another case where there were 11 houses built over 10 lots down a street. Just because there's some survey pegs doesn't mean that they're all observed, in the case of the cul-de-sac a bulldozer operator perhaps didn't see a peg and went to the next one - it's a bad mistake but it does happen.

Ortho photographs are created from a whole bunch of images and stretched to fit over a fairly rough terrain model, where there is a large distance between terrain observations the interpolation is greater giving the overall inaccuracy of +/- a few metres depending on many factors. A surveyor would have a fit if you even suggested moving 'survey accurate' plans to match an orthophoto.

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