Michael L. Binge

One of the most challenging aspects of maintaining an enterprise GIS is keeping data as current as possible to accurately reflect the actual field conditions. San Diego County recently tested and approved a program where mobile Global Positioning Units with a voice activated data entry system collect feature data in the field at regular intervals.

Global Positioning Systems have been in use for navigation and precise surveying for over twenty years. But only recently has the user technology reached the stage of development where it has become practical and cost effective in a map production and maintenance application. GPS has undergone a series of evolutions from the basic static point positioning and relative positioning to rapid static, kinematic and now real time kinematic (RTK).

1. Survey Level, The datum and controlling the base layers

This level is the base of the infrastructure and will serve as the control for all of the mapping layers and aerial photography. The equipment required to produce this level of accuracy, referred to as Survey Grade is, not surprisingly, expensive to buy and maintain. Typically, a conveniently spaced network of B Order and First Order Control Points should be established to control the mapping area. Generally the responsibility for establishing these stations lies with those agencies who maintain the National Spatial Reference System , report it to the Federal Geodetic Data Committee and publish their values. Continuous Operating Receiver Stations (CORS) are in place at various locations around the country to assist in the development and maintenance of these data sets

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For GIS purposes we generally only need to include this data from its reference source. Survey Grade equipment was used early on in for GIS pilot projects, but cost benefit analyses didn’t prove it to be the appropriate tool for general GIS applications. Post processing and data analysis is always required for this equipment. In terms of GIS, this level is generally value added. The data is frequently available for download at no charge.

2. Mapping Level, Feature data collection

Mapping level also known as "Feature Grade" GPS data collection systems have become both economical and reliable enough to apply to large scale 2D GIS mapping projects. Feature mapping at the meter and even sub-meter precision level has become an economically feasible reality with the availability of radio modem correction constants.

a: Point Positioning, requires one receiver

b: Relative Positioning, requires two receivers or a radio correction signal.

c: 3D GPS work requires two or more receivers.

Feature Coding, (The Data Dictionary) is generally included in the GPS data collection software.

Post processing (Required for higher degrees of precision.)

3. Cartographic Level (Hand Held)

This very inexpensive type of equipment is often useful to check and detect mapping blunders. It can be used to create layers that require a low level of precision very quickly and economically. It can be deployed with a very minimum amount of training.

The correct GPS tool for mapping is largely a function of map scale. With the Selective Availability encryption now deactivated (As of May 1, 2000) hand held units are capable of mapping large features on a 1:24000 scale within acceptable map accuracy standards.

The core layers of a GIS are most often constructed from elements of a system previously in use. Introducing GPS constructed layers to digitized data requires some analysis before beginning. Care must be taken in selecting the final datum and projection standards for all layers. An RMS value for all digitized data should be determined to help eliminate errors.

Still it must be remembered that old record sets are exactly that-old record sets. Digitizing and geocoding are only as good as the source data. In the end "field checking" is the only reliable method to assess the quality of the data set.

The first large project of this type in San Diego County was completed using Survey Grade technology in 1995. The Emergency Call Boxes on Highways in the County needed to be mapped. A base receiver was stationed at a high order position and another receiver was mounted to a vehicle. About 300 call boxes were mapped in less than a week using this procedure. Some post processing was done to ensure final values were within pre-analyzed result ranges.

Occupation times using this "pseudo kinematic" method were set at seven minutes. Callbox locations were spaced across 560 miles of roadways across the county. This project yielded positions accurate to about +/- 3 meters with post processing. That exceeded the pre analysis projected result of 10meters.These receivers had limited capability for point attributes. Descriptions for each location were entered by hand into a separate data base.

The remaining 400 Call Boxes were mapped with hand held receivers. The result were positions accurate to +/- 50meters and required extensive editing.

A smaller pilot project with survey grade receivers was conducted in 1997. Real Time Kinematic receivers with data loggers were employed on that project. Position accuracy was improved to nearly the decimeter level with only a few seconds of occupation, but the range from a base station to rover was limited to about 10km. Data loggers allowed simultaneous point attribute entry. Receivers were mounted on vehicles and some post processing was required.

The first pilot program to map features with this class of equipment in San Diego County was completed in 1995. It met with limited and mixed results. Using the hand held ten channel receiver once again demonstrated we could not achieve meter accuracy without employing a base station and lengthy occupation times. Data storage constraints in units of this type placed limitations on point attribute data. This class of unit also seemed more prone to multi path and obstruction errors.

In 1996 we modified one of these units with a radio modem adding a correction constant. This improved real time observation accuracies to +/- 5m. This arrangement was useful for small projects, but "blind areas" where no correction signal was available limited its potential.

In 1999 the County contracted another pilot study utilizing GPS for GIS feature collection. This time we employed a mounted feature grade receiver equipped with radio modem correction constant and voice recognition software (By Datria Systems) and collected feature and attribute data in the pilot area. The yield was these metrics.

1. Average objects per hour = 240
2. Average statements per objects = 2
3. Average objects per mile = 220
4. Average Miles Per hour = 1

With some post processing position accuracy for this survey stayed very close to the 30cm range. These data files translate easily into Arc View shape files. Five of these units will be going into full time production data collection this year.

Types of features collected in this survey were traffic signals, signs, manhole covers, lightposts, guard rails, pedestrian ramps, culverts, valves and road striping.

On the surface it would seem inexpensive hand held GPS receivers have no place in a GIS mapping environment. Our experience has proven otherwise. It is often difficult if not impossible for a GIS practitioner to verify the location of features whose position has been determined from legacy source data.

He has to rely on information from field personnel who physically visit these sites. By outfitting some of these key personnel with receivers we have been able to identify and correct errors and deficiencies in data layers for a very modest investment.

The advent of speech enabled attribute gathering was the critical element in achieving a cost benefit ratio attractive enough to commit to a full time GPS feature collection program. Allowing the user to add attributes on the fly automatically creates the framework for the metadata. This substantially reduced the processing time required to prepare the collected data for entry into the GIS in our pilot study.

· Photo by Datria Systems

Paper Number 170

GPS Data Collection and GIS Construction and Maintenance

Michael L. Binge

Land Surveyor

County of San Diego