GPS

The use of GPS has expanded considerably in the past four or five years. Applications include:

Land surveying
Intelligent vehicle/highway systems
Marine navigation
Georeferencing of satellite imagery
Mapping quality control
Service vehicle tracking
Transportation planning
Original mapping

The potential uses of GPS include any situation that requires locational information of some map feature. There are several advantages of using GPS over using existing maps or air photos to determine the location of a feature on the earth. Not all features are mapped at an appropriate scale for project requirements. For example, the location of individual trees might be necessary for a park development project. In other cases, the information might be available on a map, but the quality of the map may be in question. Mapping firms, which might have been hired to do field surveys in the past, may be replaced by the GIS analyst armed with GPS. Top

3. GPS Applications with GIS

Three important areas of use of GPS by GIS users are:

Original mapping of unmapped features
Georeferencing of imagery
Quality control.

3.1. Original mapping with GPS

As a tool for original mapping, GPS is particularly powerful. A few examples from a GPS applications contest reported by GPS World (a magazine published by GPS World, Eugene, Oregon) in May, 1992 illustrates a variety of such uses.

UNICEF used GPS to map remote villages in West Africa in order to include attributes about the villages in a GIS database. They used this information as part of their fight against the disease dracunculiasis. The disease had persisted partly because of difficulty in tracking it in villages that moved frequently and were virtually uncharted.
A small town in Montana used a GPS receiver mounted on a jeep to record the 20 mile path of a proposed road through mountainous, remote terrain. The project was completed in less than two days.
An engineering firm gathered the geographic locations of 15,000 telephone poles with GPS for a public utility GIS.

GIS users are no longer constrained to using existing maps and air photos when they need to locate uncharted attributes. Top

3.2. GPS for Image Rectification and Georeferencing

Satellite (and other remotely sensed) imagery is often a part of the GIS database. In order to align the image with other GIS data layers, it must be corrected geometrically and referenced to ground locations. Ground control points (GCPs) are used to reduce distortion and to place the image pixels in proper geographic space.
Ground control points(GCP) are coordinates collected at easily identifiable locations. They need to be visible on the image and be located in a place where coordinates can be collected. Road intersections are a common choice.
Three or more GCPs and some mathematics can be used to establish a geographically corrected grid to which the pixels in the image may be adjusted. The more precise the GCPs, the better the corrections will be. The average GCP coordinates used with satellite imagery are taken from maps of a suitably large scale as compared to the image. Even better, however, is to obtain GCPs from surveyed ground locations. GPS points of sufficient accuracy can provide excellent control points, usually superior to what could be taken from a map. Top

3.3. Using GPS for Quality Control

A third direct use of GPS with GIS is to provide data for quality control of the GIS database. The GPS receiver is used to log coordinate values associated with features in the project area. The true location of the feature compared to its location in the database will reveal the magnitude and nature of the spatial data discrepancies.
GPS is often used for mapping projects, which subsequently contribute to GIS database development. All maps require some form of ground control points. GPS units can be used to accomplish this. If current trends continue, as more land surveyors rely on GPS technology to perform surveys, GPS will touch virtually all mapping projects. The new technology that affects the mapping industry in general, affects GIS, in turn. Top

4. Conclusions

GPS is not limited to collecting point data. Kinetic GPS collects data as the receiver moves. Software attached to the GPS can close polygons, and even output the data in a format that is compatible with a particular GIS. The features that are being mapped might be linear, such as a road; a polygon, like a park boundary; or a point, such as the center of a cemetery. Once again, the value of GPS is the ability to acquire ground locations of fairly high accuracy in an efficient and cost-effective way. GPS receivers in combination with software that transforms the data into a usable form and with GIS are a powerful tool. Top

5. Equipment

We presently own three (3) Leica System 300 GPS receivers with Wild GPS sensors and controllers in a networking mode. The particulars of our receivers, the controllers, and the antennae are as follows:

3 CR-244 Controllers
2 SR-399 receivers
1 CR-399E Receiver/AT202 EXT ANTENNA
1 Mel-PC card reader for down loading of data.
Each of our systems has two 6-hours life span of batteries plus a Car Battery cable as backup. Each controller has a second one megabyte data card as a backup.

The Leica System 300 Receiver has nine channel, dual frequency- C/A code and P-code, fast sequencing receiver. The receiver operates on 9(L1) and 9(L2) continuous tracking channels. It can track up to nine satellites simultaneously. It is capable of time-marked output.
The sensor's antennae (SR-399) is microstrip and the satellite selection is automatic on the basis of the cut-off angle and satellite health criteria. It is also capable of manual satellite selection if required. The L2 tracking allows automatic switching to squaring when P-code is encrypted (i.e.S/A). The Controller, which controls the GPS sensor allows operational mode options of Static, Rapid Static, Reoccupation, Stop and Go, Kinematic or Navigation and real time capabilities. It has a large LCD which allows for 8 lines by 40 characters display, as well as satellite-tracking information, Data-logging information, as well as Real-time navigation information. The Keyboard has alphanumerics which allows easy data entry.
The Data logging device has interchangeable memory cards of 1 Megabyte capacity plus an internal memory of the same capacity. This is more than adequate memory capacity for large projects.
The Leica System 300 is capable of collecting the satellite navigation message and in measuring the pseudo-range, integrated doppler and the raw carrier observable. Single point solutions are determined on a GPS station site in real time using the pseudo-range data and are also determined during the post processing with the software package, SKI. The Wild Sensors represent state of the art advancements in the ever changing field of GPS hardware. Top

6. Projects

Our firm has been involved in a number of control projects over the years. The more relevant ones are as follows:
Port Stanley OBM control establishment. This was done using GPS and conventional means.
In 1994, the City of London had various companies including ours, help establish a 410 station control network throughout the city. At that time we developed a Validation network for a proposal which was approved by the Ministry of Natural Resources.
Our firm utilizes its own GPS equipment daily to conduct the following types of surveys; Enginnering and legal surveys, Topographical surveys, setting Control Coordinates on Reference Plans and Subdivisions, Road Profiling and Erosion studies, seismic surveys, Water boundary surveys, centreline road mapping, mapping for site specific farming (precision farming) and crop yeild management, farm tile location, as built surveys of buried services such as hydro, cable,bell and pipelines,
Patrick Levac is the head GPS Technologist and his duties involve supervising, collection of data, processing and adjusting the networks. he also has extensive knowledge of computers (hardware and software) as well as experience with the following projects during his employ with Geodetic Survey of Canada.
Deformation study on Parliament Hill in Ottawa. New Baseline establishment which involved taking measurements with a Mekometer and processing the data collected. Baseline measurements to calibrate the Mekometer and other higher precision control instruments. Processing first order data collected with astronomical instruments such as the Wild 4000 and the Kern DKM2000.
A 30 point second order control network established in OHIO for a three dimensional seismic survey. A large network was established in Honey Harbour(georgian Bay) to control shore road allowance closure reference plans. The water's edge was tied in with the use of real time GPS which provided us with in the field checks of the accuracy of the data collected. Top

Global Positioning System

1. Introduction