written by

Charles Raffensperger

November, 2019

As a location-based field data collection and management platform, one of the most critical functions SpatialWorx supports is accurate geospatial positioning in a variety of different functions and interfaces. These include feature placement and manipulation (point, line, or polygon), GPS track logging, geocoding, geofencing, search/locate, and interfaces between external or embedded systems that provide geospatial positions to SpatialWorx. Because geospatial positioning plays such a central role in many aspects of the system, most importantly in the mobile app, it's important to understand a little more about GPS accuracy and examine its role in your field solutions.

GPS Accuracy

A number of different functions in SpatialWorx (detailed below) require varying degrees of positional mapping accuracy. Additionally, the accuracy requirements for the needs of different customers can also vary widely. For example, a wildlife survey may only require positional accuracy in a range of 3-5 meters while a building site inspection may require precision down to a couple of centimeters. Because SpatialWorx is dependent on messages from a GPS receiver (internal or external to the device) and its chipset, the initial, real-time accuracy is only as good as what is transmitted from the receiver. For devices such as smartphones or tablets that may have limited GPS accuracy there are a number of external, Bluetooth-enabled receivers that can provide a much more accurate position than the primary smartphone or tablet device.

GPS receivers can generally be categorized as either Mapping grade or Survey grade and as such are designed for different purposes and levels of accuracy. Mapping grade receivers can also be broken down into Commercial grade and Differential grade.

• Mapping Grade
• Commercial Grade ( >=3 meters horizontal)
• Differential Grade ( <=1 meters horizontal)
• Survey Grade (within 1 centimeter horizontal)

Commercial (or recreational) and Differential (or professional) grade GPS units are designed for different purposes. Commercial/recreational units are designed to acquire a location fix quickly without the need for a high level of accuracy. To achieve this most of those units give a position regardless of the quality of GPS signals they receive. For some uses this is perfectly adequate. Professional GIS users (using differential grade GPS), however, typically require very accurate placement of features, often within a meter or better. A position that is tens of meters out can lead to mistakes in subsequent decision-making and for many applications an inaccurate position could be worse than no position at all. They achieve the higher accuracy with differential processing (as discussed in more detail later in this article).

The accuracy of differential grade GPS receivers varies depending upon the type of differential correction applied and the quality of the receiver and antenna (type, quality, and the number of satellite and frequencies that can be received), with external antennas typically providing the best results.

All Differential-grade GPS receivers have a horizontal positional accuracy of less than 1 meter. Most new GPS receivers with differential corrections from SBAS (Satellite Based Augmentation Systems) such as WAAS (Wide Area Augmenation System) typically have accuracies from 0.3 to 1.0 meter, depending on the quality of the receiver. Currently, the highest quality differential GPS receivers available are dual frequency units that utilize both GPS and GLONASS satellites. These coupled with a very accurate differential correction subscription will give the best differentially corrected position possible. Vertical accuracies for these GPS units are 2 - 3 times that of the horizontal accuracy, and should be used only for informational purposes.

The Commercial (Recreational) grade GPS receiver is the one that is used frequently in vehicles to assist drivers with navigating from one address to another, identifying which roads to turn on and the distance remaining to reach the destination. Some cellular phones use cell phone repeater towers to accomplish a similar level of accuracy. Generally speaking, an accuracy of about 100 feet is common for these units (sometimes better, sometimes worse).

The Differential grade GPS receiver offers the user a range of position accuracy, depending on the receiver selected. At the lower end is the GPS receiver that can collect data to within accuracy levels of 3-5 meters, or 10-15 feet. Higher grade receivers are capable of reaching accuracy levels of 0.5 - 2 meters, or 1.5-5 feet. Generally, as the level of required accuracy increases, so does the cost of the receiver and the cost of the supporting technology to process it. These receivers depend substantially on the techniques of the GPS user to collect the data as well as those used in processing the data for use in the office.

The Survey grade GPS receiver also offers the user a range of position accuracy. At the lower end, is the GPS receiver that can collect data to within accuracy levels of 1 meter, or 3 feet. Higher levels of accuracy get to less than 1 foot, within a centimeter, or even to within a millimeter. Again, as the level of required accuracy increases, so does the cost of the receiver and the cost of the supporting technology to process it. All of these Survey grade GPS receivers require post processing software and GPS data administration services to attain the "verifiable" accuracy levels.

GPS-enabled smartphones use a variety of signals to determine location such as GPS, WiFi networks, Bluetooth, or cell towers and are typically accurate to within a 4.9 m (16 ft.) radius under open sky. Next generation smartphones, in development now and slated to be on the market this year, with new chipsets from BroadComm, promise to provide accuracy levels down to 30 cm (about 1 foot). The improved accuracy is thanks to a newer GPS satellite broadcast that gives your phone additional information it can use to refine your position.

High-end users boost GPS accuracy with dual-frequency receivers and/or augmentation systems. These can enable real-time positioning within a few centimeters, and long-term measurements at the millimeter level. Some receivers and accompanying software augment primary satellite signals with information from ground-based stations to produce a more accurate position reading. There are also an array of different Bluetooth-enabled GPS receivers available that can you can use with a smartphone or tablet to get a much more accurate positional reading than would the GPS signal on the phone or tablet itself.

"Knowing where you are going is the first step to getting there."
- Kenneth H. Blanchard

Differential Correction

Every time a GPS receiver calculates its position, there is some amount of error inherent in the calculated position. Errors can be introduced from a number of sources (e.g., GPS clock errors, atmospheric conditions, the distribution of GPS satellites) over which the GPS user has little control.

Differential correction is a commonly used technique to reduce the systematic errors that decrease the accuracy of GPS positions. All differential correction techniques use correction data from a GPS base station to improve GPS locations calculated by a GPS receiver in the field (often called the "rover" because it moves with the person carrying it). The GPS base station is permanently fixed to the same location, and, as a result, its location is known with a high degree of certainty.

As part of the differential system, the base station acts as a reference location of known coordinates to determine how much error is present in the GPS system at any time. Each time the base station GPS calculates its position based on the GPS signal, the deviation from its actual location can be calculated (i.e., the error at that time can be determined). Error corrections can then be calculated almost continually and applied to data being collected in the field by a rover GPS unit in the vicinity of that base station. Differential corrections can be performed after field work (post-processing) and/or real time through a correction signal broadcast via a radio beacon or geostationary communication satellites.

The most common satellite-based differential correction of this type is a signal most GPS receivers have the capacity to receive: the Wide Area Augmentation System (WAAS), provided through the Federal Aviation Administration. Real-time differential corrections also are broadcast by the U.S. Coast Guard via radio signals and can be received and used by GPS receivers outfitted with special radio receivers.

Real Time Kinematic (RTK) satellite navigation is a technique used to enhance the precision of position data derived from satellite-based positioning systems (global navigation satellite systems, GNSS) such as GPS, GLONASS, Galileo, and BeiDou. Today all high quality, professional-grade, GPS receivers support real-time differential correction.

GPS Accuracy For Your Solution

So, with all that information about different levels of accuracy in different GPS devices it's important to note that SpatialWorx works on any iOS, Android, or Windows device (smartphones, tablets, or laptops/notebooks). If the device you're using for your field data collection solution comes equippped with a GPS chipset you'll need to determine if its positional accuracy is precise enough to meet your requirements. And that means really understanding what the data you collect or manage will ultimately be used for. How critical is the geospatial position of the features or notes on a map?

Once you have a good idea what your positional accuracy requirements are, you can determine if the native GPS capabilities of the device meet those requirements. If they don't you'll need to look at external GPS receivers that can provide the desired accuracy. There are a number of different Bluetooth- enabled receievers that have quite high accuracy and are affordable that can be taken to the field and SpatialWorx can interface directly from them to extract the GPS input stream.

The bottom line is that there are many options to choose from and it's up to you to understand what you need to make the best decision in light of your requirements and the within the framework of devices you have to work with. Happy collecting!