The accuracy of a geometry collected by the Trimble Positions software depends on a number of factors as a large number of hardware and software components are involved in collecting geometry data. The graphic below shows the factors which are the main error contributors.
GNSS infrastructure consists of navigation and positioning satellite systems (GPS, GLONASS and others). Each satellite in those systems broadcasts signals (“codes”) on two or more different carrier frequencies. The structure and quality of emitted signals and carrier frequencies and broadcasted satellite positions are among factors contributing to the position calculation error.
The GNSS receiver receives signals from satellites and uses them to calculate distances and further to calculate a receiver position (actually the position of the phase centre of a GNSS antenna). The capabilities of a receiver, like the ability to receive signals from multiple systems and multiple frequencies and to use carrier signal to measure distances have the major influence on position accuracy. The qualities of the GNSS antenna and receiver hardware and firmware also have significant effect on the precision of a result.
For each receiver type supported by Trimble Positions, expected accuracy is stated in the documentation and it includes error sources from both GNSS infrastructure and GNSS receiver/antenna. The stated expected accuracies are specified for good environmental conditions and are often given with specific confidence intervals. The following image shows a part of the Geo 6000 documentation with estimated accuracies.
Environment may have a significant effect on accuracy. Tree foliage, for example, can obscure some satellites or increase noise in their signal. Large buildings can also obscure satellites or reflect signals artificially extending distance to a satellite (“multipath error”).
Those errors are not part of the specified receiver accuracy, but Trimble Positions software will try to estimate the total position error, based on information available in the receiver at the time of measurement which may include some information about environment. Accuracies will be estimated and displayed for both positions calculated in real-time (CEA) and those that may be calculated after post-processing (PPA), based on information about a base station to be used for post-processing. The following image shows a screen shot of Trimble Positions Mobile extension showing current (CEA) and post-processed (PPA) accuracy.
Some GNSS/GIS data collection software uses PDOP (positional dilution of precision) as an estimate for position accuracy. However, PDOP takes into account only the satellite constellation and as such represents a much less reliable accuracy estimate than CEA and PPA values provided by Trimble Positions which include other sources of errors.
Trimble Positions software enables accuracy-based logging, where individual positions will be not be used to build feature geometry if their PPA value is larger than a predefined accuracy threshold for that feature type. That way a user can be confident that a collected geometry will have the required accuracy after post-processing.
Differential correction, done either in real-time or as post-processing can greatly improve accuracy as it may eliminate some error sources. However, the position calculated in differential correction is relative to the position of the correction source. That introduces new potential error sources, like the quality of a base station receiver, distance from a base station, accuracy of base station coordinates published by a provider of base data and a potential error in converting base station coordinates if specified in a different coordinate system than WGS84.
The quality of a corrected position depends on base station receiver capabilities (if they support the same GNSS systems as a data collection receiver, particularly GLONASS) and the distance between those receivers. Trimble Positions allows a user to setup base station characteristics which will be used to calculate PPA. Incorrectly set base station distance or a flag to indicate if a base station supports GLONASS can lead to PPA values which will be very different than accuracies actually achieved after post-processing. In some cases that may lead to worse accuracies than required.
Trimble Positions software always stores calculated GNSS positions in the WGS84 coordinate system. If a base station coordinate is specified in a different coordinate system it needs to be converted to WGS84 before differential correction. That conversion may require a datum transformation if the coordinate system is using a datum other than WGS84. It is very important to choose a proper datum transformation as selection of the wrong datum transformation can introduce an error of up to 1 meter. Further, many datum transformations are not very precise and their error may depend on the location where features are collected.
In tectonically active parts of the world, local datums may move several centimetres per year compared to WGS84, and it is very important to use a datum transformation that is appropriate for the particular time of measurement (“epoch”). Otherwise, additional error will be added to each calculation.
Trimble Positions software does not have access to datum accuracy information and, if a high-accuracy measurement is required, it is the user’s responsibility to find an appropriate datum transformation and to be aware of the error that a selected datum transformation will add to the position accuracy. Estimated errors after differential correction will not include a potential error due to coordinate conversion.
Trimble Positions uses classes and methods from ArcGIS for Desktop and ArcGIS for Mobile for coordinate transformations. Esri publishes lists of available datum transformations with their accuracies. Those lists should be consulted to make sure that a proper datum transformation is selected. The ArcGIS documentation also has helpful information for choosing an appropriate transformation.
Differential correction sources with global (or multi-continental) coverage (like WAAS, EGNOS and similar) use the WGS84 coordinate system so they are not affected by coordinate conversion issues. The estimated error when using those systems does not depend on local settings and is usually specified in GNSS receiver data sheets (see above).
If other measuring tools, like laser rangefinders, are used when collecting GIS feature geometry, they will introduce additional error to the calculated geometry. Laser rangefinders consist of multiple measuring devices (a laser, a digital compass and an inclinometer). Each of these devices has their own accuracy (see the image below). If a user selects a particular laser rangefinder type in the Trimble Positions Mobile extension, the accuracies specified for that type will be taken into account when calculating total feature accuracy. If offset values are specified manually or in the Trimble Positions ArcPad extension, their accuracy is not available to Trimble Positions and so will not be taken into account.
The Trimble Positions software builds feature geometries by combining GNSS positions with other measurements (if any) using specified construction (offset) types. Examples of constructions used are averaged points and bearing-distance offsets. The geometry building logic will use accuracy values (if available) for all input positions and measurements and will combine them to estimate the overall geometry accuracy. The algorithm for combining estimates will be specific for a selected construction. For example, weighted average (based on GNSS position accuracy) is used to calculate resulting point feature coordinates. GNSS positions with the best accuracy will contribute the most to the resulting coordinate, and also to the resulting accuracy estimate.
Storing geometries to GIS databases can also introduce errors. If a coordinate system used in a GIS database is different than WGS84, a coordinate system conversion (potentially including a datum transformation) will need to be executed on feature geometries. As we discussed above those can introduce errors to the resulting GIS features. Further, some older GIS databases have limited resolution for coordinate storage if feature dataset extents are set too wide. While this is not problem with Trimble Positions data collection software, customers may not be aware of that restriction.