Facilities rectification to GPS survey - At warp speed
Douglas Novy
Sanborn
1935 Jamboree Dr.
Colorado Springs, CO
80920
Email: dnovy@sanborn.com
Most municipalities have existing digital maps of utilities and parcel data. What
happens when new ortho photography is acquired and the existing digital data is not
spatially correct when overlaid with these orthophotos? Denver Water faced this
question.
Denver Water, not unlike most utility departments throughout the country, developed
its GIS with the goal of building a digital replication of existing paper sources for
more efficient map updates and data distribution. The City acquired
orthophotography and GPS data, however, the existing GIS simply didn’t meet
positional accuracy standards. Rebuild or adjust existing data was the question.
Denver Water was interested in determining the exact locations for their visible
facilities and repositioning their entire digital dataset based on these new GPS
locations. Denver Water and Sanborn combined forces to design a method of
warping the existing utility features to their exact GPS locations without affecting the
network’s connectivity and modeling capabilities. This method proved so successful,
that the Facility adjustment was followed up with the adjustment of Denver Water’s
parcel data and sixteen other coverages used as background for facility data.
Ultimately, this project has shown how careful implementation of warp
transformations can maximize efficiency and minimize manual cleanup.
History
Denver Water started hand mapping on linens in the early 1900s. Conversion to digital
mapping started in 1985 and completed in 1989 on the Informap system. This mapping
system was built with the goal of making a digital replication of existing paper sources
for more efficient map updates and data distribution. Denver Water stepped into the
GIS world in 1996 with the migration to ESRI’s ArcInfo librarian. The data could be
accessed via Denver Water’s Intranet for operations support, but engineering projects
could not use the GIS as a starting point because of the spatial inaccuracy.
Orthophotography showed Denver Water’s data was not positionally accurate.
Engineering needed positionally accurate data for design work, and they were finding
many of the mapped features were more than 80 feet from their true location. A further
need to update was due to the GIS data being incomplete. Over the years updates and
changes had been made to the actual water pipeline network, but not all updates had
been mapped in their GIS.
Pilot
The decision was made to update and rectify the data, but there was some debate as to
what would be the best method for Denver Water’s needs. A decision was made to
attempt to warp the entire network to the features’ true GPS location. Both Real-Time
Kinematic (RTK/GPS) and Differential Global Positioning System (DGPS) were tested
during the pilot to see which would be the preferred capture method, and various warps
were performed to determine the best method for rectification of the data.
The pilot project proved that the data could be adjusted satisfactorily. RTK/GPS was the
chosen method because of its higher accuracy and better reliability. The pilot also
showed the facilities would best be rectified using a rubbersheet warp. Denver Water
decided a rectified network would be awkward if the reference cadastral data did not fit.
Denver Water asked if the cadastral data could then be fit to the facilities data, so
Sanborn came up with a way to use the parameters of the facilities rubbersheet warp to
warp the reference cadastral data.
