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  • Apparent Spatial Errors and Orthophoto Building Lean

    Due to the nature of orthophotos, the location of GIS point, line and polygonal layers as compared to features in the photo can (and does) vary. There are two reasons for this.
    1. First, while orthophotos are our best spatial reference, they are still not perfectly accurate. For instance, a quality orthophoto might be "off" as much as +/- 6 feet although they are generally much better.
    2. A more significant factor is that features displayed on orthophotos are only accurate at ground level. That is, map features at ground level are at their correct horizontal location. Elevated features like tops of buildings are not rectified to their true horizontal position. This offset of the tops of buildings appears as "building lean".

      While the intent of orthophotos is to look straight down, as a practical matter, virtually all the displayed orthophoto locations are photographed at an angle. This results in "building lean" showing roofs and other elevated structures offset from their true location.


    It's common to see properly placed parcel lines and other GIS map features crossing roofs in orthophotos. You can see this effect by looking at different orthophotos of the same location which are taken at slightly different angles.



    Correctly placed parcel boundaries appear to cross roofs in this area of the "2011/04 PAG Color Orthophoto" as a result of camera angle and building lean.



    Looking at the "2008/04 PAG Color Orthophoto" instead, we see that this particular photo area was taken looking virtually straight down. There is little or no building lean and the same parcel boundaries are clearly not crossing through buildings.

    Yet, if we look at the oval island at ground level in the parking lot, then we see the parcel boundary appears in the same location at the left edge of that island in both the above "2011/04 PAG Color Orthophoto" and this "2008/04 PAG Color Orthophoto".


    Camera angle varies across the entire orthophoto area. Other areas of both of the above orthophotos show varying amounts of building lean. Neither the "2008/04 PAG Color Orthophoto" or the "2011/04 PAG Color Orthophoto" is noticably better or worse with respect to areas of building lean.

    As always, remember that our maps and orthophotos are not accurate to survey standards as explained in our disclaimer.

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