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  • Rectifying Vector Data Layers with Orthophoto Imagery

    By overlaying existing vector GIS data with orthophotos, spatial errors in our vector GIS data are quite apparent. Suddenly, spatial errors in our GIS database that were otherwise unknown and of no concern are all too visible to laymen and experts alike.

    This is a good time to (re)read our disclaimer which talks about inaccuracies that are inherent in our GIS data.

    MapGuide Maps

    Orthophotos are available on the "Main MapGuide Map" and the "Orthophoto Map" on our primary MapGuide page and many other MapGuide map pages.

    Map vector and point layers aren't perfectly registered with orthophoto imagery. Where there are positional differences, the orthophotos are generally more accurate spatially.

    The exception to this is the Geodetic Control Points layer which is quite accurately registered to the orthophotos. Be aware that these control points are not section corners and are not necessarily aligned with section corners. In other words, the control points may not appear to be in the "right" place, but probably are. This and more is explained on the Geodetic Control Points page. Further, because the section grid is based on various sources and has other spatial errors, the section grid layer may not be entirely accurate on the orthophoto map.

    Accuracy and the Nature of GIS Data

    Our GIS database was initially developed prior to availability of orthophotos and was never intended to be perfectly accurate. Perfect accuracy is close to impossible with GIS.

    When data is entered into a GIS system, there are tradeoffs between accuracy, cost, sources of information, and the intended use of the data. Our GIS database is a planning database that shows relationships accurately with features registered and located as well as possible within the constraints at the time the data was entered -- and our GIS data library has been built continuously since 1988. Users want the data to be accurate within inches, but as a practical matter this is too expensive and not needed by virtually all the users of the data.

    When more accuracy is needed our GIS data is linked to source documents such as Subdivision Plat Maps, Assessor Record Maps, or construction plans that may provide more accurate spatial geometry within limited areas.

    Unfortunately, parcel boundaries and other vector map features are not necessarily visible in the orthophotos as there may not be anything physical (or at least anything visible) defining the boundary in an orthophoto. Even where a boundary such as a fence can be seen, it may not represent the legal boundary. Further, street pavement borders are usually not representative of the right-of-way.

    Within these limitations, we manually register vector data layers with the orthophotos. Spatially correcting our vector layers is an ongoing project of continuous refinement.

    On the other hand, it's possible you are seeing errors we aren't aware of. If this explanation doesn't adequately explain spatial errors you are seeing, then please let us know what is wrong in more detail. Certainly there are errors that have nothing to do with spatial accuracy concerns. We want to know about these errors and fix them. Send e-mail using the Send Feedback link describing the location and nature of the error(s) you see.

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    Geographic Information Systems

    201 N. Stone Ave., 9th Fl.
    Tucson, AZ 85701

    Phone: (520) 724-6670
    Fax: (520) 791-6588


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