THE USE OF GPS FOR GIS APPLICATIONS
K. Ramadan
Assistant Prof., Transportation Dept.,
Faculty of Engineering,
Alexandria University,
Alexandria,Egypt.
E-Mail "Ramadan@Alex.Eun.Eg"
Abstract
During the last decade, the demand for GIS in management and design applications has greatly increased. The realization of a GIS in many application areas still suffers from data acquisition problems. To be of value of the user, a GIS must be updated regularly, so that the information in the data base correctly represents the real world. However, the acquisition of up - to date GIS data by conventional survey techniques is prohibitive in cost and has therefor limited the applicability and usefulness of GIS to potential users. Described in this paper is an attempt to overcome this problem. The present work focuses on the use of GPS for GIS applications which has entered a new age with the operational capability of GPS system as well the developments in GPS receiver technology and processing strategies. The current status of GPS methods and accuracies are reviewed in the context of GIS applications. A combination of GPS survey and GIS techniques were employed to construct a digital database for use in the study Area and to get a topographic map for the same area.
1. Global Positioning System; Technology review
1.1 Definition
GPS is a satellite-based navigation and positioning system developed by U.S. Department of Defence. It uses a ground receiver to determine location by the triangulating between several satellites in known positions. The GPS ground receiver measures the distance between itself and each satellite by recording the amount of time it takes for a radio signal to travel from the satellite to the receiver and by knowing the speed at which the signal travels (186,000 miles per second).By knowing the exact locations of the satellites, and its exact distance from each of those satellites, the GPS receiver is able to calculate its own position precisely [11].
1.2 The impact of GPS on Surveying
GPS was conceived primarily to serve military user requirements and cannot as such satisfy all of civilian user expectations [12]. In the beginning of the 1980's, the first civil GPS users could be found in Geodesy [14]. The NAVASTAR Global Positioning System (GPS) is revolutionizing the surveying and mapping industry. It has provided a new tool for performing very precise, homogeneous geodetic control surveys, but more importantly, it is being used in a multitude of applications to provide positioning information during the collection of Geographic Information System (GIS) attribute data [5].
The high precision of GPS carrier phase measurements, together with appropriate adjustment algorithms, provide an adequate tool for a variety of tasks in Surveying and Mapping [13]. The impact of the Global Positioning System (GPS) on the Surveying and Mapping community has already become significant, while its use in Geographical Information Systems (GISs) is just beginning to be realized [4]. The GPS has already made a major impact into the field of Surveying and Georeferencing [3].
1.3 Technology review
The GPS is divided into three segments: space segment, control segment, and user segment. Table 1 defines the inputs, the functions, and products of each segment.
The space segment consists of the satellite vehicles which its configuration consists of 24 satellites in six orbital planes with an inclination of 55 degrees.Twenty one satellites are active and three are spares. The satellites are approximately 20,000 km above the earth and complete each orbit of the earth in 12 hours.
The control segment is responsible for operating the global positioning systems. It consists of five control stations, spaced almost evenly around the world. All five stations track the GPS signals for use in controlling the satellites and predicting their orbits. Three stations are capable of transmitting data back up to the satellites, including new ephemerides, clock corrections and other broadcast message data and command telemetry. The station located at the Falcon Air Force station near Colorado Springs is the Master Control Station.
Table 1 The space, control, and user segments of GPS
Segment |
Input |
Function |
Product |
Space |
Navigation Message. |
Generate and Transmit Code, Carrier Phases,and Navigation Message. |
P Codes, C/A Codes, L1, L2 Carrier, Navigation Message. |
Control |
P Code, Observations, and Time (UTC). |
Produce GPS time, Predict Ephemeris, and Manage Space Vehicles. |
Navigation Message. |
User |
Code Observation, Carrier Phase Observation and Navigation Message. |
Navigation Solution and Surveying Solution. |
Position, Velocity, and Time. |
Every satellite transmits two frequencies for positioning: 1575.42 MHz and 1227.60 MHz. The two carriers, called L1 and L2, are coherent and modulated by various signals. The C/A, P, and Y codes are pseudo-random noise (PRN) codes consisting of a series of plus and minus ones that are modulated onto the L1 and L2 carriers. The range (distance) to each satellite is determined by either interpreting the PRN codes or by measuring the phase shift of the carrier wave. If the PRN codes are used a pseudo-range is determined by measuring the time shift between the received code and a similar code generated within the receiver. Measuring the phase shifts produces a more precise range, however, it is a major problem to determine the full number of wavelengths between the receiver and the satellite. A second problem with using the phase measurements is to ensure that the integer count is maintained throughout the observations (i.e. there are no cycle slips).
The user segment consists of the receivers and associated computer software for receiving the satellite signals and computing position, velocity and time. There are many methods of survey techniques which have been developed to take advantage of the GPS's capabilities. Some of them are static, Rapid static, Pseudo static, semi kinematic, kinematic and navigation differentially corrected.
We've got distance measurements to some satellites whose positions we know exactly. We'll see how that translates into fixing our position. Suppose a receiver determines that it is 23,000 kilometres from a particular satellite. That one measurement really narrows down where in the universe that receiver could possibly be. It tells us it is somewhere on the surface of an imaginary sphere that's centered on that satellite and that has a radius of 23,000 kilometres. If it measures its distance to a second satellite and finds that it's 26,000 kilometres from that one, that further narrows down where it could be in space. The only places that are both 23,000 km from the first satellite and 26,000 km from the second satellite are where those two spheres intersect. That intersection is a circle of points. A third measurement adds a third sphere which will intersect the circle formed by the other two. The intersection occurs at two points and so with three measurements, the receiver has narrowed down its position to just two points in all the universe. A fourth distance measurement would go through one of those two points but in actual practice we may not need that fourth measurement because one of the two points will be unreasonable (i.e., thousands of kilometres away from earth). There is another reason for that fourth measurement, however, The fourth measurement gives us a way to make sure our receiver's clock is truly synchronized with universal time.
A number of errors effect GPS. Some are beyond the control of the user and others can be minimized by the use of careful planning and survey techniques. Table 2 lists these errors. It is difficult to cite specific precision's because of many variables involved. However, typical results obtained by many users are quoted in Table 3 below.
Table 2 Sources of Errors in GPS
Satellite orbit errors. Satellite clock errors. Receiver clock errors. |
System errors. |
Ionosphere. Troposphere. |
Atmospheric errors. |
Blocked signals. Multipath. |
Obstructions. |
Selective Availability. Anti Spoofing. |
System degradation. |
Poor network design. Survey blunders. |
Human errors. |
Table 3 GPS Precision
Precision |
Method |
5 mm + 1 ppm |
Static. |
1 cm + 1 ppm |
Rapid static. |
1 cm + 2 ppm |
Pseudo static. |
2 cm + 1 ppm |
Semi-kinematic. |
2 cm + 1 ppm |
Kinematic. |
2 - 5 m |
Dynamic (differential corrections). |
25 m |
Dynamic (no SA). |
100 m |
Dynamic (with SA). |
1.4 Differential Global Positioning System (DGPS):
GPS positioning is affected by ephemeris and satellite clock errors due to Selective Availability (SA), atmospheric delays, receiver noise, vehicle dynamics and multipath. DGPS techniques provide measurement corrections which eliminate (or at least diminish) most of these errors and high accuracy positioning is thus feasible.The resulting accuracy of DGPS positioning depends on the accuracy of the measurement corrections supplied from the reference station as well as data latency [1]. In "differential" mode surveyors or using GPS try to make measurements down to a centimeter [8]. GPS in differential mode is capable of collecting data more quickly and with smaller crews in comparison to the existing method of collecting data with a total station [7].
2. Geographical Information Systems
2.1 Definition
A Geographical Information System (GIS) is a computer - based mapping and information system designed to collect, store, edit, display and retrieve graphic and alphanumeric information from a spatial database. A GIS is designed for the collection, storage, maintenance and analysis of objects and phenomena in which geographic location is an important characteristic or a critical factor to the analytical and decision - making process.
GIS applications usually start by building a land base. Land base information is typically acquired through a combination of Surveying, Photogrammetry or Remote Sensing, and capturing data from existing maps via scanning or digitizing. Some applications allow use of scanned, controlled photographic images as backgrounds with vector data displayed on top. Once the geographic and facility features have been entered, the system keeps of their spatial relationships, allowing a variety of analysis to be made.
2.2 Benefits of GIS
GIS offers a number of benefits:
3. The relationship between GPS and GIS
3.1 Integration of GPS and GIS
By integrating GPS with GIS we are able to perform "Real World" digitizing for the capture of physical data and objects [6]. Any device that generates coordinates and outputs data in the format recognized by a GIS can be used as a digitizer. This included in a particular GPS receivers and photogrammetric plotters [10]. GIS, GPS and remote sensing technologies in increasing numbers to improve financial returns, boost land productivity and ease compliance reporting [9].
3.2 Link between GPS and GIS
Direct link between GPS and GIS is that the GPS receiver is treated as the cursor of a digitizer. It is linked to the GIS through a software module similar to a digitizer controller. Data goes directly into the GIS filing system. A GPS receiver is really the digitizer cursor, while the earth is the digitizing "table". It is like the case of a photogrammetric plotter except the "stereo model" used is the real world. Comparing conventional digitizing with GPS digitizing is shown in Table 4.
Table4: Comparing conventional digitizing with GPS digitizing
Conventional |
GPS |
Accuracy dependent on scale.
Suitable for massive data collection. Tracking speed controlled by user. Suitable for objects that can be seen from aerial photography. |
Accuracy not dependent on scale. Suitable for selective updating. Tracking speed controlled by speed limit and traffic. Can also be used for small objects.
|
Indirect link between GPS and GIS is that the GPS receiver records data in its own storage using its own format. This data is later translated to various GIS data formats. The translated data is then loaded into the GIS filing system.
3.3 GIS data base benefit of GPS
One of the greatest advantages of GPS for GIS applications is that data is collected quickly and accurately with a common reference system. With data logging GPS receivers the coordinates, time, and other attribute information may be collected and then exported to a GIS data base with no manual digitizing operation. Since GPS provides a common reference system, data from GPS sources and sources rectified with GPS will register with each other and with GPS data collected in the field. This is an important feature, especially when SPOT raster imagery is combined with vector data in a modern GIS data base [2].
4. Application
Data of this application was captured using GPS as a real digitizer for a certain district in Canada. The data was obtained with an Ashtech dual frequency receiver. The accuracy is indicated by the root mean square (RMS) in the original file, Table 5. This table shows data of 327 points in the format of longitude, latitude and height. The original data file was converted to the format of cartesian coordinates (x, y and z). The converted data file is shown in Table 6. The first line in this table should be ignored as it represents the size of the World Geodetic System (WGS84) ellipsoid which is used in GPS. Data is shown in these tables in a concise form, but the completed data is kept on a floppy disk.
The used postprocessing software is called "Prism". It is a product of the Ashtech company. The software can perform static, kinematic and pseudo-kinematic GPS Surveying. It can also perform the GPS network adjustment. A secondary part of the software is to perform all kinds of datum transformation. The cartesian format data was analyzed by using Surfer version 4, Golden software to get a topographic map. The final results are shown in Figure 1 and Figure 2.
Table 5 The original data file
Site |
Latitude |
Longitude |
Height |
RMS |
0001 0002 0003 0004 0005 0006 0007 0008 0009 . . . . . . 0064 0065 0065 0065 0065 0065 0066 0066 0066 0066 0066 0067 |
N 37.42310289 N 37.42305554 N 37.42307199 N 37.42326890 N 37.42326717 N 37.42326768 N 37.42327101 N 37.42327173 N 37.42324213 . . . . . . . . N 37.42284694 N 37.42284414 N 37.42284410 N 37.42284405 N 37.42284405 N 37.42284404 N 37.42307257 N 37.42307252 N 37.42307253 N 37.42307251 N 37.42307258 N 37.42308156 |
W 122.07834827 W 122.07829298 W 122.07819454 W 122.07844870 W 122.07841530 W 122.07834632 W 122.07824212 W 122.07819664 W 122.07810466 . . . . . . . . W 122.07853337 W 122.07891215 W 122.07891233 W 122.07891223 W 122.07891230 W 122.07891231 W 122.07923086 W 122.07923086 W 122.07923081 W 122.07923086 W 122.07923090 W 122.07923090 |
-29.0643 -29.1192 -29.2013 -30.0803 -30.0986 -30.1903 -30.2493 -30.2664 -30.1976 . . . . . . . . -30.9675 -31.2053 -31.2083 -31.2073 -31.2033 -31.2063 -31.3333 -31.3283 -31.3303 -31.3283 -31.3273 -31.3273 |
0.023 0.023 0.023 0.023 0.023 0.023 0.023 0.023 0.023. . . . . 0.026 0.026 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 0.025 |
Table 6 The converted data file
ASHTECH POINTS FILE
PROGRAM: PRISM v2.1.00 Nov 11 1996
CREATED FROM: C-File CSHORE3.163
SYSTEM: GEOG Geographic Coordinate DATUM: WGS84 World Geodetic Sys. 1984 ELLIPSOID: WGS84 World Geodetic Sys. 1984 SEMI-MAJOR AXIS: 6378137.000
INVERSE FLATTENING: 298.2572236
UNITS: METER METER
POINT X Y Z SITE
00001 6378137.000 0.000 0.000
00002 -2693402.768 -4297258.465 3854772.057 0001
00003 -2693402.767 -4297258.461 3854772.060 0001
00004 -2693402.767 -4297258.459 3854772.059 0001
00005 -2693402.769 -4297258.459 3854772.059 0001
00006 -2693402.771 -4297258.460 3854772.057 0001
00007 -2693400.294 -4297263.733 3854767.850 0002
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
00324 -2693469.096 -4297217.187 3854768.004 0066
00325 -2693469.091 -4297217.188 3854768.004 0066
00326 -2693469.096 -4297217.188 3854768.003 0066
00327 -2693469.097 -4297217.183 3854768.010 0066
00328 -2693468.775 -4297216.669 3854768.802 0067
Figure 1 A contour map for the data file.
Figure 2 Surface of the contour map.
5. Conclusions
GPS is a rapidly maturing technology that is being used in many GIS applications. It is moving away from a control survey tool to one that is used for positioning of GIS entities. New receivers and software systems are appearing in the market place at an exponential rate.
In conclusion, a combination of GPS survey and GIS techniques were employed to construct a digital data base for use the study area and to get a topographic map for the same area. GPS positioning can be used to set up an overall control network. Digital land base information is preferable, since it can be input directly into the GIS, eliminating the costs and potential in accuracy of data conversion.
The integration of GPS and GIS has allowed the updating to be more efficient and will, in future, allow surveys to be based on a spatial reference system thus eliminating the need to set up a costly network. GPS is an operational system that will benefit the users of GIS data bases in collecting accurate geographic coordinates for creating, updating or maintaining their own GIS data bases.
Finally, it can be said that GIS and GPS technology has continued to develop at a rapid pace. The use of GPS in GIS can be summarized in these points: as a real - world digitizer, for data retrieved and analysis, for tracking moving objects and for ground truthing.
6. References