President, Applied Field Data Systems Inc.
What is GPS
GPS or Global Positioning System is a constellation of 27 satellites orbiting the earth at about 12000 miles. These satellites are continuously transmitting a signal and anyone with a GPS receiver on earth can receive these transmissions at no charge. By measuring the travel time of signals transmitted from each satellite, a GPS receiver can calculate its distance from the satellite. Satellite positions are used by receivers as precise reference points to determine the location of the GPS receiver. If a receiver can receive signals from at least 4 satellites, it can determine latitude, longitude, altitude and time. If it can receive signals from 3 satellites, it can determine latitude, longitude and time. The satellites are in orbits such that at any time anywhere on the planet one should be able to receive signals from at least 4 satellites. The basic GPS service provides commercial users with an accuracy of 100 meters, 95% of the time anywhere on the earth. Since May of 2000, this has improved to about 10 to 15 meters due to the removal of selective availability.
GPS technology offers several advantages: First and foremost, the service is free worldwide and anyone with a receiver can receive the signals and locate a position. Second, the system supports unlimited users simultaneously. Third, one of the great advantages of GPS is the fact that it provides navigation capability.
Limitations of GPS
As with any technology, GPS also has some limitations. It is essential that the users are aware of these limitations.
One must recognize that a GPS receiver gives a location reading, which is subject to some inherent errors some under our control and some outside our control. Unless specific steps are taken to improve the accuracy, even with the Selective Availability (SA) off stand-alone receivers can be as much as 15 meters off.
In order to obtain a GPS position reading, one needs to occupy the point. Often one cannot get there (maybe you don’t want to cross a highway with heavy traffic) or you do not want to get there (wildlife etc.).With GPS, if you cannot occupy a point, you cannot obtain the GPS reading.
Even if one can reach the point, the area may be covered with a canopy (thick forest) where GPS signals cannot reach and therefore cannot get the reading. GPS needs clear view of the sky.
The elevation readings from GPS receivers are not very accurate. Even with differential GPS, the elevation readings can be 2 to 3 times worse than horizontal readings.
There are several sources of error that reduce the accuracy of the GPS receivers. Some are random in nature and beyond our control but some errors can be controlled.
As the satellites are transmitting signals from an elevation of about 12000 miles, the radio signals pass through earth’s atmosphere before reaching the receiver. The signals have to pass through ionosphere and are delayed. This delay does affect the accuracy obtained by the receiver. Keep in mind that the signals travel at speed of light 186000 miles per second and even a 1/100th second delay can put one off by 1860 miles.
The satellite orbits get affected by gravitational pulls, which cause disturbance in their orbits and create errors in the position.
The receivers themselves can cause some errors. For example if there is faulty clock in the receiver, it will impact the readings.
The multipath error applies to the placement of the receiver. This happens when the receiver gets a direct signal from a satellite and also a reflected (bounced off) signal from a large object (trees, high rise buildings etc.) around the receiver.
How is accuracy improved – DGPS
Some receivers have the capability to automatically correct for some of the above errors. However even with these types of receivers one expects to get 5-10 meters accuracy. Some users feel that with the removal of SA, they can average the readings and get sub meter accurate readings. This is not true. The readings may improve but several inaccurate readings will not always give you an accurate reading. In order to improve the accuracy further, one needs to use a technique called differential GPS (DGPS). Note that this will increase the cost of the system.
The principal of DGPS is simple. If 2 receivers are placed close to one another, around 100-200 kms they will be subject to the same amount of errors and travel through the same atmospheric conditions. So one uses 2 receivers- one at a known point (base) while the other receiver is collecting the data in the field (rover). The base receiver at the known point stores the position data in the memory or on a PC, while the rover stores the data from the field in its onboard or external memory. The computer compares the second by second data from GPS unit at the base with the actual known point data at the base station and determines the amount of error. When the data from the rover is downloaded in the PC, the software applies the corrections to the rover data and corrects the rover readings. This method is called the post processing method. This method, while providing good accuracy has some limitations and disadvantages:
- One needs 2 receivers (thereby raising the cost) or access to some base station data from a location within 200 kms from the rover.
- This method also does not provide you with real time navigation capabilities.
- Frequently, if the satellites tracked by the base And rover units are different, the readings will not be corrected.
- The other factor to consider in this method is the fact that for every hour spent in the field to collect the data, one needs to spend about an hour in the office post processing this data.
Instead of using the post processing method, one can now utilize the real time correction method. In this case instead of storing the base station data and processing on the PC, the error is calculated in the receiver at the base and broadcast. The U.S. Coast Guard offers one such system and if one has a GPS receiver with an appropriate beacon receiver, one can receive the corrections in real time and accurate GPS readings are displayed. While the service is free, it has a limited range. This range is heavily dependent on the topography of the area. Therefore, this is not a solution for everyone.
The second real time system is offered by private companies who are transmitting the correction signals from a satellite. This allows for far better coverage all over the world. But the service ranges around $800.00 to $5,000.00 per year for various areas in the world. Monthly service is also available.
The last 2 methods offer the capability of real time accurate readings and therefore good navigation capabilities. Also one does not have to spend time in the office to correct the field data. What is GIS
GIS stands for Geographic Information System. It’s a computer-based system designed for storing, analyzing, updating, manipulating and displaying spatial data. GIS can be viewed from 3 perspectives-the map, the data and the spatial analysis.
The map view focuses on the ability to create and present information in a cartographic manner. This enables the presentation of information in a visual manner and assists in building user knowledge.
Data is an important component of GIS. It provides users with a tool to capture, manage, query and analyze data from various sources. There are 2 different components of data stored in GIS.
- Component which defines location a spatial component such as:
points- discrete features ie, a tree, building, traffic sign
lines – connected locations ie, roads, rivers
areas – which have length and width ie, lakes, perimeter of a building etc.
A descriptive component describing the feature ie, length of road, height of tree etc.
The power of GIS lies in its ability to identify relationships between features based on their locations and their attributes. GIS also allows one to view these relationships in maps, charts, tabular forms etc. In GIS one has the ability to create different layers of data. For example, one can create layers for roads, traffic signs, accidents, buildings, fire hydrants, water lines, etc and once the layers are created one can superimpose different layers and analyze the relationship between various features and their attributes. For example, if one wants to repair a pothole, one can quickly turn on the layer for potholes, roads and water lines and determine if repairing the pothole would entail digging the road under which a water line is running.
In order to analyze the data one requires relevant and current information, which can be obtained from various sources. Some data can be obtained from existing sources eg population, census, roads etc. However, other data specific to a project may not be readily available and may have to be collected for a specific project with GPS. Once the data is collected in GPS receivers, it can be directly transferred to a GIS System. This data will have the spatial location as well as feature/attribute data attached to it.
Integration – GPS/GIS Technology
GPS is a powerful tool providing a unique position of a specific feature. With this information, one can navigate back to it. However, one cannot relate this “feature position” to any other “feature position” unless one is standing at the site and other features are visible.
GIS by itself provides great analysis capability but to achieve that one needs plenty of good data. As explained earlier, some data is available but a lot of other data needs to be collected to allow the full capabilities of GIS to be utilized.
Combining the GPS data with GIS allows for greater capabilities than what GPS and GIS can provide individually. With the combination of two technologies one is able to display the “FIELD/ACTUAL SITE” on a PC and make informed decisions. There is no need to make specific site visits or review several documents/drawings. Also, another benefit of the integration is the fact that the data can be shared by unlimited users in various departments for their own specific needs and analysis.
Another important advance in this technology has been the introduction of a software which allows bringing into GIS not only GPS position information but a digital picture. With this software, one can study relationships between features but also view actual photographs of the features right on your PC.
Figure 1 shows chemical discharge points obtained with GPS. In GIS software they appear as points with XY positions.
Figure 2 shows same points superimposed on map of waterways (obtained from existing sources, Texas Rivers.shp). Now one can visualize the close proximity of some of the hazardous chemical discharge points to waterways.
Figure 3 shows above but with roads (obtained from existing sources, Texas Roads.shp) and now one can relate the existence of chemical discharge points in relation to waterway & roads.
Figure 4 shows all of the above but with population density and now one can observe the potential impact of accidental chemical discharge on population.
One can keep on adding “layers” of data and visualize relationships between chemical discharge points and other features.
One can start asking “What if” questions once the data is displayed. For example, one can ask, “Which waterways or how much population will be impacted by a accidental chemical discharge from a specific point?”
- Figure 5 shows cultivated land and wildlife sanctuaries – data collected by GPS.
- Figure 6 shows the points superimposed with waterway layers.
Figure 7 shows the above with the flood layers displayed and this indicates which cultivated lands and sanctuaries may be impacted by the floods.
One can decide by looking at this map which areas are prone to flooding and therefore not suitable for wildlife sanctuaries and agriculture.
Figure 8 shows a background of roads (obtained from existing sources, Roads.shp).
Figure 9 shows a new layer of “Pot holes” generated by GPS.
Figure 10 shows existence of pipeline along the roads.
Just by looking at this map one can see the potential of damaging the pipeline by the crew digging around the potholes to fix them.
In all the above examples, one has been able to display the maps on the computer and ask “What if” questions. There has been no need to go and visit specific sites to analyze this data. One does not have to visit the site in cold or hot weather, be too close to traffic/ wildlife, scribbling notes on pieces of paper. Integration of the 2 technologies brings the “actual site” on to your PC and allows one unlimited analysis capabilities.
Pitfalls of Integration
Combination of GPS/GIS technology is limited by the amount of data. As explained earlier one needs lot of good data for conducting analysis. While some data is available, a lot of data has to be generated by the users for their use. Sometimes collection of field data could turn out to be time consuming and expensive.
Data collected must be accurate and meet the correct formats. For example one needs to make sure that all the “layers” of data displayed are in same units (feet/meters) and the projections and datums match. Without this the analysis will not be correct.
The integration of GPS with GIS brings the real world to the desktop. What could take days to visit a specific site and analyze can now be performed on your desktop. The power of GPS/GIS is immense and application are unlimited and varied in all areas such as agriculture, environmental, defense, natural resources, health, business etc. As the price of hardware and software comes down I see the potential of this integration to grow tremendously in country like India.