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