Facilities rectification to GPS survey - At warp speed
Work plan project specifics
Step one was for Denver Water to first clean and update their data as well as possible
before delivery to Sanborn. This would minimize potential problems from trying to
adjust an incomplete network. Updating the data with network changes Denver Water
already had would also minimize the number of new features found by Sanborn’s
survey crew.
Denver Water then divided the city into 16 areas with 62,000 features over 179 square
miles and posted data to an Internet project collaboration web site for download.
Sanborn downloaded Denver Water’s data and imported it to Sanborn’s proprietary
software format (APS). This software was chosen because APS Has the Capacity to:
- Import/export Various Platforms
- Build and propagate Attribution
- Perform numerous warp transformations
- Easily manipulate large amounts of graphic data
Knowing the reference cadastral data would need to be warped based on the warp of
the facilities data, Sanborn came up with an innovative procedure. Before any data was
processed, a pair of cells with matching ids were built on parcel corners. Sanborn
called these cells “warp seeds” and would use the distance between seed pairs after the
first warp to calculate the second warp. Denver Water uses 10 digit unique ids on their
water features. It was felt that these ids could be shortened to five or six digits to
minimize errors when the survey crew keyed in the ids. Sanborn assigned this
shortened “alias id” to each feature prior to survey. Sanborn then exported the features
for GPS capture, alias id text, road boundaries and names to DXF for loading into
survey field equipment.
A collaborative effort between Denver Water employees and Sanborn crews was used
to capture the GPS locations. The Denver Water employees were particularly helpful in
identifying features in areas where the existing data had greatest spatial inaccuracy,
greatly reducing the number of features not found by Sanborn’s survey. New features,
when found, were collected and delivered to Denver Water for future inventory.
Upon completion of an area, the GPS survey crews would send the point coordinates to
Sanborn & Denver Water, via the project collaboration web site.
Sanborn imported the collected points to APS. Sanborn’s next step was to warp only
the facility data without compromising the cadastral data. To accomplish this, the water
network, associated text, and one set of warp seed points were separated into a
separate file from cadastral data.
A Rubbersheet warp transformation was used to adjust the water network to the new
GPS locations. The rubber sheet warp transforms three or more source points to the
same number of control points with zero residual errors. The transformation is
piecewise linear: the algorithm forms a triangular irregular network (TIN) from the
source points and maps points in a triangle using a local affine transformation defined
by the source/control point pairs that make up the three triangle vertices. Note that the
transformation is continuous inside the TIN, but is non-continuous outside the TIN
(transformations that fall outside the TIN are extrapolations). In other words, the TIN
should contain the boundary of the point set being transformed. Basically a rubbersheet
is best visualized as the name implies. The water network is treated as if drawn on a
sheet of rubber. By moving each feature to sit atop its true location, the water pipes and
neighboring features are stretched and skewed based on the movement of surrounding
features, but the network is not be broken.
The network was warped using only water valve points. Butterfly and conduit valves are
offset from their surface features, so their true locations could not reliably be captured
from the surface. Although captured, Fire hydrants were also omitted from the water
network warp. Hydrants had been digitized into Denver Water’s data based on a
different set of requirements and were found to be much farther off of their true location
than were water valves. The fear was that features neighboring hydrants would be
overly skewed if hydrants were used as warp locations. Once the network was warped,
the hydrants were programmatically moved to their true locations by reassigning their
xyz coordinates to the true GPS coordinates. Their pipes were then “snapped” to
reconnect with the hydrants.
Hydrants were next visited to reconnect hydrant branches perpendicular to the main,
per Denver Water’s requirements.
Sanborn’s next step was the warp of cadastral data based on average warp of the water
network. The cadastral data had been previously separated, with cadastral annotation
and one set of warp seed points, into its own file. The warp seed points in this file were
warped to the points moved during the water network’s rubbersheet warp. This
cadastral warp was done using a Helmert 2d transformation. Using this warp, the
cadastral data was moved based on the overall average facilities warp for a given area.
The Helmert_2d transformation is one of the most conservative warp transformation
algorithms. It uses a single variable scale factor (homogeneous shrinkage or
expansion), rotation, and translation, so the integrity of the parcels would not be
compromised. Although the final warped locations would not be as exacting as those of
the rubbersheet warp, square parcels would remain square and blocks of parcels would
be moved near where they needed to be. It was assumed there would be some manual
cleanup to the cadastral data.
Warp seeds, both the original locations and the locations after the network warp were
saved in a separate location saved for use later in warping project-wide coverages.
A manual review and cleanup of entire dataset was performed after this second warp.
Denver Water’s criteria included:
- Horizontal accuracy of plus or minus 5 cm
- 100% connectivity must be maintained
- Edge matching must be maintained along delivery area boundaries
- Exact feature count must be maintained
To facilitate review of potential problem areas, Sanborn implemented a PAR (problemaction-
resolution) on-line process with Denver Water. This enabled Denver Water to
review live data with Sanborn, and create resolutions to inconsistencies without the time
and expense of face to face meetings.
Once all potential problem areas were reviewed, the data was converted back to ArcInfo
coverage format, with the new features Sanborn’s survey found separated from existing
features through attribution. Data was then delivered to Denver Water’s QC contractor
and Denver Water via the Internet project management site.
30 QC steps were completed including surveying 3 random checkpoints per section to
check for accuracy of collected points. Upon passing the 30 QC steps, acceptance or
rejection was recommended (no areas have been rejected to date).
