GIS Inspection using GPS and Digital Camera
Solution Components
Trimble GPS PathFinder
The Trimble ProXL GPS was used to collect the coordinates of the manhole. A data dictionary
with pre-defined pick-lists, allowing convenient attribute selection was developed for data input.
A serial connection cable was used to download data from the GPS memory card to the computer
at the beginning of the project. Towards the end of the project, Trimble ASPEN application was
used to allow the operator to directly input field data into a portable computer. ASPEN data
transferring was much more efficient than the serial data connectivity, and will be used for all
future projects.
Coordinates Projection
The project requires the delivery of Arc/Info data using a Baltimore Grid coordinate system. The
Baltimore Grid uses the Baltimore Monument as the (0,0) central point. The City provided
control points for data projection. An Arc/Info AML was created to convert State Plane
coordinates based GPS data to the Baltimore Grid.
Post Differential Correction
The field data collected with the Trimble ProXL GPS needs to be corrected for positional
accuracy. The Department of Defense currently broadcasts signals from the satellites with small
random spikes in the signals. These spikes need to be identified and corrected. To accomplish
this, the GPS Base station data from locations within Maryland or surrounding areas must be
used to correct the field collected GPS data positions. This base station data is collected over a
known reference point, sorted by date and time. This data is published on the Internet and is
easily downloaded. Then using routines supplied by Trimble, the DXF files of the GPS collected
data can be corrected within 3 meters. If greater accuracy is required, a base station can be set
up within a few miles of the project and then sub-meter accuracy can be accomplished.
Data Conversion
After the fieldwork is processed for positional accuracy, the Trimble Pathfinder program data is
exported to an Arc/Info file format. The Arc/Info file is also converted to a MicroStation format
with attribute pointer reserved as a part of the MicroStation element data. The reason for this
conversion is to enable database attribute review and verification while creating design drawings
within Micro Station environment. The Arc/Info to Micro Station conversion can be
accomplished using an AML program from Arc/Info environment. CADDScan also created a
utility to read Trimble-generated MicroStation files reserving database linkages.
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