When collecting data 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. For those who 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 are capable of using various navigation satellite systems, while GPS receivers can only use the navigation satellite system called the Global Positioning System. Due to the widespread use of GPS receivers, the term 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 that the GPS receiver uses—and, consequently, the more signals it receives—the more accurate it is. This is also true for global navigation satellite systems: the more systems the receiver uses (and the more signals it receives), the more accurate it is. Today, multiple navigation satellite systems are available. However, usually the more accurate a GPS receiver is, the more expensive it is and the more difficult it is to carry in the field. You can also improve the accuracy of your position information through differential corrections of the data, covered later in this topic.
There are many GPS receivers available on the market; however, not all of them work directly with Collector. To use a GPS receiver with Collector, the receiver must support the output of NMEA 0183 sentences. NMEA is the data specification standard that Collector uses to communicate with GPS receivers. NMEA messages contain lines of data called sentences. Collector derives GPS information such as latitude, longitude, height, and fix type by reading specific sentences in NMEA messages. For more information, including details on the NMEA sentences Collector uses, receivers supported on iOS, and the GPS receivers Esri has tested with, see GPS receiver support.
Most high-accuracy GPS receivers support the NMEA sentences that Collector uses; however, it's recommended that you check whether your receiver supports these NMEA sentences in the receiver's user manual before you try to connect it to Collector.
Not all receivers that support the output of NMEA sentences are configured to do so out of the box. The device's user manual should explain how to configure it to output NMEA.
Once you have chosen a receiver, your data collectors need to use a location profile to define the coordinate system of the data from the receiver and apply a datum transformation to the data if one is required. If you are using differential corrections, and the location provided to you is in a different coordinate system than your map, you'll need to provide datum transformation information to your data collectors. See Datum transformations later in this topic for details. To ensure high-quality data, you'll also want to provide your data collectors with the required accuracy that they should set in the app, as well whether they should enable GPS averaging and, if so, the number of positions to average.
To improve the accuracy of your positions, consider using a GPS receiver that supports differential corrections. Differential correction technology further improves accuracy by leveraging reference stations, which are also known as base stations. A reference station is another GPS receiver that is established on a known location. The reference station estimates its location based on satellite signals and compares this estimated position to the known position. The difference between these positions is applied to the estimated GPS position calculated by the user's GPS receiver, also called the rover, to get a more accurate position. The user's receiver must be located within a certain distance from the reference station for differential corrections to occur. Differential corrections can be applied in real time in the field or when postprocessing data in the office.
Differential corrections can be provided by public or commercial sources. One of the most widely used and publicly accessible real-time correction sources is the Satellite-Based Augmentation System (SBAS), which is also commonly referred to as the Wide Area Augmentation System (WAAS) in the United States. It is free to use SBAS, but your GPS receiver must be capable of accepting correction messages in the Radio Technical Commission for Maritime Services (RTCM) format. Using commercial correction services typically requires a subscription and may also require purchasing a particular type of GPS receiver that can receive these correction signals. See the Differential GPS Explained article in ArcUser Online for more information.
The locations provided by your receiver may be in a different coordinate system than the map you are using in Collector. If this is the case, use a datum transformation to maintain the accuracy of your data.
When you receive a location from a GPS and use it to add or update a feature, that location is in geographic coordinates that are referenced to a geographic coordinate system (GCS). A GCS includes an angular unit of measure, a prime meridian, and a datum that is based on a spheroid. Your map also has a coordinate system, determined by the basemap it uses, which may be a GCS or a projected coordinate system (PCS). If the location and map have different coordinate systems, the location being added or updated must be transformed to match the map's coordinate system. This conversion process is called a datum transformation. While there are both horizontal and vertical datum transformations, Collector only supports horizontal transformations.
Since coordinates, maps, feature services, and databases all have coordinate systems, a datum transformation (or transformations) should occur any time that your coordinate system doesn't match that of where the data is being used: between the GPS and the map, the map and the feature service, or the feature service and the geodatabase. Whenever a datum transformation is done, error is introduced into your locations. By choosing the right coordinate systems for your map, feature services, and databases, you can limit the number of transformations, as well as the error introduced each time. See ArcGIS Geographic and Vertical Transformation Tables for details on the error introduced through various transformations.
If the coordinate systems don't match and no datum transformation is provided, the data is used as it is provided. As a result, its location won't align with the locations of other data already in place. Similarly, you'll see incorrect positions if you provide the wrong datum transformation.
In Collector, data collectors can create a location profile prior to collection and specify the specific datum transformation to use. While creating a location profile, the coordinate systems of both the GPS data and the map are specified. Based on this information, users are presented with only relevant transformation methods, with the recommended method listed first. As the map author, you should provide to your data collectors the coordinate systems of the GPS and the map, as well as the transformation method they should use.
If you are using one of the basemaps provided by Esri on ArcGIS Online, it is in the WGS 1984 Web Mercator (Auxiliary Sphere) [WGS84] coordinate system. Similarly, WGS84 is the default coordinate system for GPS data received in Collector. If you are using an ArcGIS Online basemap and the default location provider, no datum transformations are necessary in Collector.