Accuracy of GPS data

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A good way to get involved in the OpenStreetMap project is to upload GPS track logs (also named GPS traces). Recorded by your GPS device, the typical track is a record of your location every second, or every meter. The collected data can be displayed as a background of thin lines or little dots within the map editor. These lines and dots can then be used to help you add map features (such as roads and footpaths), similar to tracing from aerial imagery. GPS Satellite NASA art-iif.jpg
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The accuracy of GPS data depends on many factors. For example, the quality of the GPS receiver, the position of the GPS satellites at the time the data was recorded, the characteristics of the surroundings (buildings, tree cover, valleys, etc) and even the weather. This page gives a basic introduction as to how GPS works and describes some of the key issues related to accuracy.

How GPS works

The Global Positioning System (GPS) is a satellite navigation system that provides location information anywhere on or near the Earth's surface. It comprises a number of satellites in orbit above Earth. Each satellite continually transmits messages that include the time the message was transmitted, and the satellite position. On the ground the GPS unit receives these messages and, by comparing the time at which the message was received (on its internal clock) against the time which the message was transmitted, it works out how far away it is from each satellite.

In order to calculate its location the GPS unit must receive messages (signals) from a minimum of four satellites. Consider the following:

Sphere2-intersect.svg A GPS unit receives signals from a number of satellites. Lets call them "Green", "Red" and "Purple". On receiving each signal it calculates its distance from each satellite.

If the GPS unit only receives a signal from the "Green" satellite, then it can only determine that its location lies somewhere on the sphere of all locations that are the same distance from the "Green" satellite (as shown as the green sphere in the diagram above).

Now consider the case when the GPS unit receives signals from both the "Green" and "Red" satellite. As before it determines its distance from each satellite. As we have received two signals we can narrow the location down to those points where the two individual distance spheres intersect. This means the location must be somewhere on the blue circle as shown in the diagram.

Sphere3-intersect.svg By introducing a third satellite we can further narrow the location down to two points (as shown as yellow dots). Only one of these points will be on the Earth’s surface and therefore we can discard the other. With just three satellites we have trilaterated (similar to triangulation) our location. In practice a fourth satellite is needed to improve accuracy (particularly altitude accuracy) due to errors in measuring the precise time at which each signal was received.
For more information see this video or animation.


Factors affecting accuracy

Given a basic understanding of how GPS works, this section describes some of the key issues effecting the accuracy of GPS. These include:

  • The GPS receiver unit (quality);
  • the position of the satellites at the tie the recording was made; and
  • the characteristics of the surrounding landscape.

GPS receiver

There are many GPS devices that you can use to record track logs. This includes dedicated GPS loggers, to smartphones with built in GPS, and everything in between. As you might expect, the quality of the GPS receiver can greatly effect the accuracy of your recorded track logs. The following areas are of particular importance.

  1. Aerial
    Most obviously, a good aerial is required in order to detect the message signals coming from the GPS satellites. The strength of a GPS signal is often expressed in decibels referenced to one milliwatt (dBm). By the time the signals have covered the 22,200km from satellite to Earth's surface, the signal is typically as weak as -125dBm to -130dBm, even in clear open sky. In built up urban environments or under tree cover the signal can drop to as low as -150dBm (the larger the negative value, the weaker the signal). At this level some GPS devices would struggle to acquire a signal (but may be able to continue tracking if a signal was first acquired in the open air). A good high sensitivity GPS receiver can acquire signals down to −155 dBm and tracking can be continued down to levels approaching −165 dBm.
  2. Number of channels
    As described in the #How GPS works section above, a 3 satellite system, in theory provides all the data you need to calculate a reasonably accurate location. However clock inaccuracies mean that this theory resides to the textbooks. In practice signals must be received from a minimum of four satellites in order to correct for errors. The more the better. Although early GPS receiver were limited to the number of satellites they could track at any one time, modern GPS receivers have enough "tracking channels" to follow all satellites in view. More channels are however still helpful to reduce the time it takes to get an initial fix (cold start) and to reduce power consumption. For more reading see here.
  3. Position algorithms
    To calculate the distance the GPS receiver is from each satellite, the receiver first calculates the time that this signal has taken to arrive. It does this by taking the difference between the time at which the signal was transmitted (this time is included in the signal message) and the time the signal was received (by using an internal clock). As the signals travel at the speed of light, even a 0.001 second error equates to a 300km inaccuracy of the calculated distance! To reduce this error level to the order of meters would require an atomic clock. However, not only is this impracticable for consumer GPS devices, the GPS satellites are only accurate to about 10 nano seconds (in which time a signal would travel 3m). It is for precisely this reason why a minimum of four satellites is required. The extra satellite(s) is used to help correct for the error. Although rarely publicised, it is therefore important that your GPS receiver includes good error correction algorithms.


Position of satellites

Signals from a varying number of "in view" satellites to determine your position on the earth

As noted above, generally the more satellites used in calculating your position the greater the level of accuracy. As the GPS satellites orbit around Earth, the number of satellites in view (under optimal conditions) naturally fluctuates. This can be seen in the animation on the right. Obviously the position of the satellites is completely out of our hands, however it is worth recognising this as a factor influencing accuracy. For example, this is one of the many reasons two GPS tracks recorded on separate days will differ. If you have time, it may be worth recording a track twice (or more) and averaging the results.

Some GPS receivers can display the number of satellites currently in view and their positions on a radar type diagram. On some receivers this can be prominently found in the within the standard menus, however on others it may be within a "hidden" or "debug" menu. Unfortunately with hundreds of GPS receivers available, it is impossible to provide documentation for all devices - please refer to the manual that came with your device or try searching online. Smartphone apps with this "satellite view" feature are shown in the monitoring features table for both iOS and Android based phones.

See also the section on #Enclosed spaces below.


Your location

Reflections signal weakening

Error caused by reflections and shading under tree cover.

GPS requires a direct line of sight between the receiver and the satellite. When an object lies within the direct path, accuracy suffers due to reflections and weakening of signals. This is particularly problematic in urban environments, within valleys and on mountain slopes. In all three situations, the objects (buildings and the Earth itself) are substantial enough to completely block the GPS signals. When weak signals are received, they may have been reflected off buildings and the surrounding landscape. Reflections generate multi-path signals arriving with a small time delay at the receiver. This results in inaccurately calculated position.

Even when the object is less substantial (tree cover, car roof, your body), reflection and weakening of signals may still occur. This can sometimes be observed when viewing your recorded GPS track logs on top of aerial imagery. In the image on the right, the true position of the footpath follows the shadowy area in the forest. However, as the GPS receiver enters the forest (walking from east to west), it can be observed that reflections cause the recorded track to incorrectly shift slightly to the south.

When carrying a GPS device, generally, the higher the antenna is fixed, the better the reception. Good positions include the shoulder strap or the top pocket of a backpack, mounted on top of a cycle helmet, or a roof antenna on a car.

Enclosed spaces

Highly clustered satellites can give large errors.
A disperse set of satellites improve accuracy.

Being in an enclosed space, such as a steep sided valley or a high rise urban environment, reduces the area of sky visible to the GPS receiver. This causes two problems. Firstly, it reduces the number of satellites that are in direct line of site of the receiver, therefore breaking the "the more the better" rule described above. Secondly, it prevents the GPS device from receiving GPS signals from a disperse set of satellites - that is, the satellites used to calculate your location are clustered within a small area of the sky.

Highly clustered satellites can result in large positional errors, up to several hundred meters. Although there is little that can be done to improve the situation in enclosed spaces, it is worth keeping an eye on your GPS device so that you are aware of when the signal quality drops. Look for a "satellite view" diagram (as shown in the images on the right) on your device.

For more information, or if your device also reports a "DOP Value", you may wish to read wikipedia:PDOP.

Troubleshooting GPS reception

In vehicles

If you plan to record a track from a vehicle, get a very good fix before you enter it. This is especially true for newer trains, where you might well never get one otherwise.

When do you know reception is good?

A 3D fix is not a sufficient criterion of quality. The PDOP is an indicator of the precision of the GPS measure (Position Dilution of Precision). If it is higher than 6, you can consider that you do not have a good fix. Under 4, it is good enough for OSM tracking. Less than 2 means you have a very good fix. The quality of the DOP depends on the GPS capacity of correcting the satellite's signal. You can have a good DOP with only a 2D fix.

See also