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Pima County Geographic Information Systems

GIS Overlay Inaccuracies on Pictometry Oblique Photos


While parcel boundaries and other GIS layers displayed over oblique photos are helpful for identifying things and finding your way around, they are subject to potentially significant and misleading distortion and parallax errors. Geometry points and segment vertices are displayed at calculated underlying terrain elevations. However, connecting lines between the points are not draped over the terrain and appear displaced as a result of parallax errors when there are elevation changes between the points.

In the oblique screen captures below, the default north view shows a parcel boundary clearly passing through a house while the west view shows the same line where we might expect it, comfortably between the two properties (but still not in the true GIS location). The north view also shows a significant angle in parcel 10818035B's northern boundary where it's perpendicular to another boundary. That angle is much smaller in the west view.

The GIS parcel geometry displayed in the orthophoto view is not subject to parallax distortion at ground level. The orthophoto view shows the parcel boundaries in their true GIS locations (subject to the relatively small inaccuracies in the source GIS layer and photo coordinates). Notice that the property line displayed with angles in the oblique north and west views is shown to be a straight line in the orthophoto view and as depicted in the subdivision plat map.

While this example is a more extreme case in a hilly area, it illustrates how any elevation differences in oblique views may result in misinterpreting overlaid GIS data. Displaying GIS data on oblique photos is helpful, but it needs to be used with caution and wary eye.


Oblique view looking north (default) - Shows misleading parcel boundary through house


Oblique view looking west - Better, but still "off" somewhat in this case.


Orthophoto view looking straight down - Most accurate parcel boundary representation


Subdivision plat map


Example With Overlapping Parcel Lines

The samples above show parcel geometry using Pima County's GIS parcel layer which is not available to the public on Pictometry. Public users use the "US Parcels" layer provided by Pictometry from another source. This US Parcels layer appears to have extraneous overlapping (duplicated) parcel lines in many cases where there are vertices at collinear parcel boundary endpoints.

While these overlapping parcel lines with vertices appear as a one line in orthophoto views, they appear displaced at different locations in oblique views with varying terrain elevation. These artifacts resulting from overlapping parcel lines further illustrate how GIS layer data may be displaced from its true location when viewed on Pictometry oblique imagery.


Oblique view looking north with public "US Parcels" layer - Shows distortion of overlapping parcel lines.


Orthophoto view looking straight down with public "US Parcels" layer - Shows overlapping parcel lines as one.