The accuracy required when collecting a location depends on the type of project you are working on.
For some projects, such as census enumeration, points in the vicinity of a building may provide enough information. For other projects, such as environmental sampling, the location collected must be within a few centimeters of the actual location.
When collecting a location using a device's location service, position information can be determined from various sources, such as GPS, cellular, Wi-Fi, or Bluetooth networks. The accuracy of these sources varies, and the device's location service is not always reliable. To perform data collection that requires better accuracy and reliable quality control, using a professional-grade or high-accuracy GPS receiver is usually the best option.
Global Navigation Satellite System (GNSS) is the standard generic term for navigation satellite systems. GNSS receivers can use various navigation satellite systems, while GPS receivers can only use the Global Positioning System (GPS) navigation satellite system. Due to the widespread use of the term GPS to refer to both types of receivers, GPS is used as the generic term in this help.
A high-accuracy GPS receiver precisely calculates geographic locations using information from GPS satellites. The accuracy of these receivers ranges from submeter to centimeter, depending on their ability to track and process satellite signals. GPS satellite signals are transmitted on different frequencies. The more frequencies the GPS receiver uses—and, consequently, the more signals it receives—the more accurate it is; this is also true for GNSS: the more systems the receiver uses (and the more signals it receives), the more accurate it is. Numerous navigation satellite systems are available. Typically, however, the more accurate a GPS receiver is, the more expensive it is and the more difficult it is to carry in the field.
Note:High-accuracy receivers that connect via cable or Bluetooth can only be used with the Survey123 field app. The Survey123 web app can only show the location provided by the device's internal position source.
Take the following steps to prepare a survey that will be used in the Survey123 field app with a high-accuracy receiver:
- Prepare your survey to record GPS metadata
- Configure location accuracy threshold
- Configure location quality expressions
Prepare your survey to record GPS metadata
Firstly you need to decide whether the person completing the survey needs to see the metadata or not. If they do, you can add note or decimal questions that show the values on the form. If they do not need to see the data, but you want it stored in the feature layer with the other answers to questions, you add them to your spreadsheet as hidden decimal questions.
You use the pulldata function to extract the metadata from the geopoint question.
As an example, this expression can be used to populate a decimal question with the GPS accuracy.
The accuracy value returned by the receiver at the time to geopoint question is updated, will be stored in the record. For a list of all location parameters that can be used, see Extract geopoint values.
Configure location accuracy threshold
By default, Survey123 does not check for accuracy in the position values it collects. If position accuracy is important to your survey results, you can define an accuracy threshold with location averaging. This can be done by entering a numerical value in the body::accuracyThreshold column of your geopoint question; this value will be the threshold of accuracy beyond which values are no longer captured when averaging a location.
Configure location quality expressions
The location accuracy threshold is a single number, a minimum value for the horizontal accuracy of a given location. To warn a user that their location doesn't meet other more complex quality requirements, or constrain them from submitting that location, an expression can be used. There are two columns (and their associated message columns) where these expressions can be entered:
- The constraint and constraint_message columns can be used to prevent the user from capturing a location in a geopoint question.
- The bind::esri:warning and bind::esri:warning_message columns can be used to display a warning to the user if the expression is not met. If the expression is not met, the bar containing the coordinate value at the top of the geopoint question will turn yellow. In this case, the user can still capture the location.
An expression can be created in either of these columns using any questions from the survey, but typically questions or parameters related to the survey location would be used.
For more information, see Location quality expressions in Geopoints.