In other words, what is your receiver's positioning error? The answer depends in part on your receiver. If you used a hundred-dollar receiver, the radius of the circle you drew might be as much as ten meters to capture 95 percent of the points. If you used a WAAS-enabled, single frequency receiver that cost a few hundred dollars, your error ellipse might shrink to one to three meters or so. But if you had spent a few thousand dollars on a dual frequency, survey-grade receiver, your error circle radius might be as small as a centimeter or less.
In general, GPS users get what they pay for. As the market for GPS positioning grows, receivers are becoming cheaper. Still, there are lots of mapping applications for which it's not practical to use a survey-grade unit. For example, if your assignment was to GPS 1, manholes for your municipality, you probably wouldn't want to set up and calibrate a survey-grade receiver 1, times. How, then, can you minimize errors associated with mapping-grade receivers? A sensible start is to understand the sources of GPS error.
If you want a readable and much more detailed treatment of this material, I recommend Jan's book. See the bibliography at the end of this chapter for more information about this and other resources. Douglas Welsh personal communication, Winter , an Oil and Gas Inspector Supervisor with Pennsylvania's Department of Environmental Protection, wrote about the challenges associated with GPS positioning in our neck of the woods: " In a city with tall buildings and the deep valleys of some parts of Pennsylvania, it is hard to find a time of day when the constellation will have four satellites in view for any amount of time.
In the forests with tall hardwoods, multipath is so prevalent that I would doubt the accuracy of any spot unless a reading was taken multiple times. The arrangement of satellites in the sky also affects the accuracy of GPS positioning. The ideal arrangement of the minimum four satellites is one satellite directly overhead, three others equally spaced near the horizon above the mask angle.
Imagine a vast umbrella that encompasses most of the sky, where the satellites form the tip and the ends of the umbrella spines. GPS coordinates calculated when satellites are clustered close together in the sky suffer from dilution of precision DOP , a factor that multiplies the uncertainty associated with User Equivalent Range Errors UERE - errors associated with satellite and receiver clocks, the atmosphere, satellite orbits, and the environmental conditions that lead to multipath errors.
The combination of these two components of the three-dimensional position is called PDOP - position dilution of precision. Since satellite orbits are known, PDOP can be predicted for a given time and location. Various software products allow you to determine when conditions are best for GPS work. I have had a chance to use GPS survey technology for gathering ground control data in my region and the biggest challenge is often the PDOP position dilution of precision issue.
The problem in my mountainous area is the way the terrain really occludes the receiver from accessing enough satellite signals. During one survey in Colorado Springs I encountered a pretty extreme example of this. Geographically, Colorado Springs is nestled right against the Rocky Mountain front ranges, with 14, foot Pike's Peak just west of the city.
My GPS unit was easily able to 'see' five, six or even seven satellites while I was on the eastern half of the city. However, the further west I traveled, I began to see progressively less of the constellation, to the point where my receiver was only able to find one or two satellites. If a degree horizon-to-horizon view of the sky is ideal, then in certain places I could see maybe degrees. There was no real work around, other than patience.
I was able to adjust my survey points enough to maximize my view of the sky. From there it was just a matter of time Each GPS bird has an orbit time of around twelve hours, so in a couple of instances I had to wait up to two hours at a particular location for enough of them to become visible. So the PDOP value was never as low as I would have liked, but it did drop enough to finally be within acceptable limits.
Next time I might send a vendor out for such a project! This activity will introduce you to the capabilities of the interface and will prepare you to answer questions about GPS mission planning later.
The online tool that you will use in this exercise requires that Microsoft Silverlight be installed on your machine. Silverlight does not run under all Web browsers. If you do not have Silverlight installed for the browser you are using you be prompted to install it. A variety of factors, including the clocks in satellites and receivers, the atmosphere, satellite orbits, and reflective surfaces near the receiver, degrade the quality of GPS coordinates.
The arrangement of satellites in the sky can make matters worse a condition called dilution of precision. A variety of techniques have been developed to filter out positioning errors.
Random errors can be partially overcome by simply averaging repeated fixes at the same location, although this is often not a very efficient solution. Systematic errors can be compensated for by modeling the phenomenon that causes the error and predicting the amount of offset. Some errors, like multipath errors caused when GPS signals are reflected from roads, buildings, and trees, vary in magnitude and direction from place to place. Other factors, including clocks, the atmosphere, and orbit eccentricities, tend to produce similar errors over large areas of the Earth's surface at the same time.
Errors of this kind can be corrected using a collection of techniques called differential correction. Differential correction is a class of techniques for improving the accuracy of GPS positioning by comparing measurements taken by two or more receivers. Here's how it works:. The locations of two GPS receivers--one stationary, one mobile--are illustrated below in Figure 5. The stationary receiver or "base station" continuously records its fixed position over a control point.
The difference between the base station's actual location and its calculated location is a measure of the positioning error affecting that receiver at that location at each given moment.
In this example, the base station is located about 25 kilometers from the mobile receiver or "rover". The operator of the mobile receiver moves from place to place. The operator might be recording addresses for an E database, or trees damaged by gypsy moth infestations, or street lights maintained by a public works department. The base station calculates the correction needed to eliminate the error in the position calculated at that moment from GPS signals.
The correction is later applied to the position calculated by the mobile receiver at the same instant. The corrected position is not perfectly accurate, because the kinds and magnitudes of errors affecting the two receivers are not identical, and because of the low frequency of the GPS timing code. For differential correction to work, fixes recorded by the mobile receiver must be synchronized with fixes recorded by the base station or stations. You can provide your own base station, or use correction signals produced from reference stations maintained by the U.
Federal Aviation Administration, the U. Coast Guard, or other public agencies or private subscription services. Given the necessary equipment and available signals, synchronization can take place immediately "real-time" or after the fact "post-processing".
First let's consider real-time differential. WAAS-enabled receivers are an inexpensive example of real-time differential correction. Federal Aviation Administration, c. WAAS base stations transmit their measurements to a master station, where corrections are calculated and then uplinked to two geosynchronous satellites 19 are planned.
WAAS signals compensate for positioning errors measured at WAAS base stations, as well as clock error corrections and regional estimates of upper-atmosphere errors Yeazel, The DGPS network includes some 80 broadcast sites, each of which includes a survey-grade base station and a "radiobeacon" transmitter that broadcasts correction signals at kHz just below the AM radio band.
DGPS-capable GPS receivers include a connection to a radio receiver that can tune in to one or more selected "beacons. Stephanie Brown personal communication, Fall reported that where she works in Georgia, "with a good satellite constellation overhead, [DGPS accuracy] is typically 4. According to surveyor Laverne Hanley personal communication, Fall , "real-time kinematic requires a radio frequency link between a base station and the rover.
I have achieved better than centimeter accuracy this way, although the instrumentation is touchy and requires great skill on the part of the operator.
The opposite has also happened, where I wanted to record positions and had a radio link back to the base station, but the GPS geometry was bad.
Kinematic positioning can deliver accuracies of 1 part in , to 1 part in , with relatively brief observations of only one to two minutes each. For applications that require accuracies of 1 part in 1,, or higher, including control surveys and measurements of movements of the Earth's tectonic plates, static positioning is required Van Sickle, In static GPS positioning, two or more receivers measure their positions from fixed locations over periods of 30 minutes to two hours.
The receivers may be positioned up to km apart. Only dual frequency, carrier phase differential receivers capable of measuring the differences in time of arrival of the civilian GPS signal L1 and the encrypted military signal L2 are suitable for such high-accuracy static positioning.
Users upload measurements in a standard Receiver INdependent EXchange format RINEX to NGS computers, which perform differential corrections by referring to three selected base stations selected from a network of continuously operating reference stations.
NGS oversees two CORS networks; one consisting of its base stations of its own, another a cooperative of public and private agencies that agree to share their base station data and to maintain base stations to NGS specifications. The map above shows the distribution of the combined national and cooperative CORS networks. Notice that station symbols are colored to denote the sampling rate at which base station data are stored. After 30 days, all stations are required to store base station data only in second increments.
This policy limits the utility of OPUS corrections to static positioning although the accuracy of longer kinematic observations can also be improved. The context is a CompassData project that involved a carrier phase differential GPS survey in a remote study area in Alaska. The objective was to survey a set of nine ground control points GCPs that would later be used to orthorectify a client's satellite imagery. So remote is this area that no NGS control point was available at the time the project was carried out.
The alternative was to establish a base station for the project and to fix its position precisely with reference to CORS stations in operation elsewhere in Alaska. The project team flew by helicopter to a hilltop located centrally within the study area. With some difficulty they hammered an 18 inch 5 rebar into the rocky soil to serve as a control monument. After setting up a GPS base station receiver over the rebar, they flew off to begin data collection with their rover receiver. Thanks to favorable weather, Chris and his team collected the nine required photo-identifiable GCPs on the first day.
The centrally-located base station allowed the team to minimize distances between the base and the rover, which meant they could minimize the time required to fix each GCP. At the end of the day, the team had produced five hours of GPS data at the base station and nine fifteen-minute occupations at the GCPs.
As you might expect, the raw GPS data were not sufficiently accurate to meet project requirements. The various sources of random and systematic errors that contribute to the uncertainty of GPS data are considered elsewhere in this chapter. In particular, the monument hammered into the hilltop was unsuitable for use as a control point because the uncertainty associated with its position was too great.
The project team's first step in removing positioning errors was to post-process the data using baseline processing software, which adjusts computed baseline distances between the base station and the nine GCPs by comparing the phase of the GPS carrier wave as it arrived simultaneously at both the base station and the rover.
The next step was to fix the position of the base station precisely in relation to CORS stations operating elsewhere in Alaska. The following steps will guide you through the process of submitting the five hours of dual frequency base station data to the U. For information about OPUS, go here. Download the GPS data file. If you can't download this file, contact me right away so we can help you resolve the problem. Explaining all the contents of the file is well beyond the scope of this activity.
For now, note the lines that disclose the antenna type, approximate position of the antenna, and antenna height. You'll report these parameters to OPUS in the next step. To do this, you'll need to determine a the uncorrected position originally calculated by the base station, b the corrected position calculated by OPUS, and c the mark-to-mark distance between the original and corrected positions. Links to these utilities are listed below.
Determine the position of the base station receiver prior to differential correction. Determine the corrected position of the base station receiver. Look for the latitude and longitude coordinates and ellipsoidal height that are specified relative to the NAD 83 datum. They should be very close to:. Calculate the difference between the original and corrected base station positions. NGS provides another software utility to calculate the three-dimensional distance between two positions.
Positions are a fundamental element of geographic data. Sets of positions form features, as the letters on this page form words. Positions are produced by acts of measurement, which are susceptible to human, environmental, and instrument errors.
Measurement errors cannot be eliminated, but systematic errors can be estimated and compensated for. Land surveyors use specialized instruments to measure angles and distances, from which they calculate horizontal and vertical positions. The Global Positioning System and, to a potentially greater extent, the emerging Global Navigation Satellite System enables both surveyors and ordinary citizens to determine positions by measuring distances to three or more Earth-orbiting satellites.
As you've read in this chapter and may know from personal experience , GPS technology now rivals electro-optical positioning devices i. This raises the question, "If survey-grade GPS receivers can produce point data with sub-centimeter accuracy, why are electro-optical positioning devices still so widely used? I also enjoyed a fruitful discussion with an experienced student named Sean Haile Fall Here's what they had to say:.
In general it may be said that the cost of a good total station EDM and theodolite combination is similar to the cost of a good 'survey grade' GPS receiver. While a new GPS receiver may cost a bit more, there are certainly deals to be had for good used receivers. In such a case the EDM is less expensive. Still, that is not the whole story. Remember, you need line of sight with the EDM. Of course, if a topo survey gets too large, it is more cost effective to do the work with photogrammetry.
And if it gets really large, it is most cost effective to use satellite imagery and remote sensing technology. Now, lets talk about accuracy. It is important to keep in mind that GPS is not able to provide orthometric heights elevations without a geoid model. Geoid models are improving all the time, but are far from perfect. The EDM on the other hand has no such difficulty. The conventional equator is defined to be at right angles to the z-axis.
Being at right angles to the z-axis, the x-axis and y-axis therefore lie on the conventional equator. The x-axis points to the prime meridian zero longitude , which is slightly offset from the Greenwich meridian for historical reasons. The y-axis is then defined to be at right angles to the x-axis, thus forming a right-handed coordinate system. Note that the height defined in this fashion is geometrically the distance normal to the surface of the ellipse.
This is not generally the same thing as physical height above mean sea level. To compute a physical height, it is further necessary to correct for the height of the ellipsoid above the geoid Torge, Such corrections are provided by models as a function of latitude and longitude.
Deviations between the geometric ellipsoid and the physical geoid can exceed m in some locations. Given that uncertainty in geoid models can far exceed the uncertainty in GPS Cartesian position, it is usually preferable to use GPS height above the ellipsoid unless the application demands a physical height. For example, it is typically sufficient to use ellipsoidal height for monitoring height variation in time. However, physical height may be needed for large-scale engineering projects involving the flow of water.
User access to the WGS 84 reference system is enabled by the transmission of data to the user on the orbits and atomic clock times of the GPS satellites. Data in the navigation message is computed by the GPS control segment by least-squares estimation of the GPS orbit trajectories and atomic clock times while holding fixed the WGS 84 reference frame coordinates of official GPS tracking stations.
While the WGS 84 reference frame coordinates are improved from time to time, the reference system WGS 84 maintains its name.
Since its initial development, the WGS 84 reference frame has become considerably improved by making it consistent with a much higher accuracy reference system ITRF discussed in the next section. Therefore coordinates that are specified in WGS 84 are now consistent with ITRF, though the degree of coordinate accuracy in either frame is another issue. ITRF is updated from time to time and is labeled by the year of the last data that contributed to the frame definition. In order to accommodate plate tectonic motion, ITRF is defined not only by position coordinates of reference frame stations but also by velocity coordinates.
Position coordinates in ITRF are regularized coordinates, in the sense that they do not represent the actual current position of a point, but rather the position after correcting for all physical effects specified by ITRS IERS Conventions, The largest effect is caused by solid Earth tides , which are the deformations of the solid Earth caused by the gravitational pull of the moon and sun.
A lesser effect which is important nearer the coastlines is ocean tidal loading. Models must be specified in ITRS to be consistent with the definition that the origin is at the center of mass of the entire Earth system, including the ocean and atmosphere. What are 3 ways to identify location? What are the three ways we can identify a location?
How is place different from location? How does location affect the location of the place? What are the three types of regions? What is a cultural region example? What makes a region two examples? What are the 7 cultural regions? What are main cultural areas? What are the 3 types of culture? If the number is negative, it represents south of the Equator. The line of longitude is read as 2 degrees 2 , The coordinate for the line of longitude represents east of the Prime Meridian because it is positive.
If the number is negative, it represents west of the Prime Meridian. The line of latitude is read as The line of longitude is read as 2. The image above shows the location of the Statue of Liberty on Google Maps.
The coordinates given for its location are:. Just to recap, the Decimal Degree DD coordinates does not have the letters N or S to signify whether the latitude coordinate is above or below the Equator. Neither does it have the letters W nor E to signify whether the longitude coordinate is to the west or east of the Prime Meridian. This is done through the use of positive and negative numbers.
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