Configuring GPS collection settings

If you plan to use a GPS device to collect new features in the field, you can configure properties of the GPS collection that apply to a specific feature layer. You can also choose whether field-workers can override the settings that you chose for them.

It is important to set a level of quality for the shape of GIS features that are collected using a GPS receiver that matches or betters the accuracy requirements you have for your data. Quite often, that quality bar can be different for each feature layer that is updated.

Understanding how a GPS works is essential when choosing appropriate GPS data collection settings for your field projects. If you are new to GPS data collection, it is recommended that you spend time learning GPS principles and how to use GPS receivers for the purpose of data collection.

There are several factors that can influence the quality of a GPS position received, such as the quality of the GPS receiver that is being used, your current location and environmental conditions, and atmospheric conditions. The GPS Quality settings listed on the GPS Status page of the field application will provide you with some indication of the quality of the positions you are receiving—fix type, number of satellites, PDOP, and HDOP.

DOP, which stands for dilution of precision, is a measure of the position of the satellites: how many you can see, how high they are in the sky, and the bearing toward them. This is often referred to as the satellite geometry. As the satellites move, their geometry also changes. A low DOP value generally indicates a higher probability of accuracy.

DOP is divided into components because the accuracy of the GPS system varies. PDOP stands for position dilution of precision. The best PDOP would occur with one satellite directly overhead and three others evenly spaced about the horizon, and the value would be 1.

Good satellite geometry lowers DOP values.

When collecting new GIS features using GPS, ArcGIS Mobile applies a GPS quality filter to each position that is read from the GPS receiver. If the position meets the quality filter specified for the feature layer, it will be used in calculating the location that you are capturing.

A GPS quality filter is set for each editable map layer using the GPS tab. The GPS quality filter is based on two key factors:

• GPS fix type
• PDOP

GPS fix type

Depending on the GPS receiver used in the field and the type of GPS data collection you are to perform, you might want to filter the positions received from the GPS device based on a certain fix type. For example, you might want to ensure that only differentially corrected GPS locations are used in the averaging process and ignore all autonomous read positions.

PDOP

In addition to the type of fix that you are receiving, you may also want to filter positions based on the geometric strength of the GPS satellite configuration. PDOP is a numeric value representing the amount of error in the position read. Setting a PDOP value as a part of the GPS quality filter will ensure that only positions with a PDOP equal to or less than the set value will be used to create the average. The default value is 6.

Depending on the accuracy requirements for your data collection projects and the environmental conditions that are faced by your field-workers, you can choose whether the quality filter settings can be modified within the GPS Collection Settings form in the ArcGIS Mobile application itself. By default, quality filter settings can be updated in the application.

GPS collection methods

There are two primary GPS collection methods available in the ArcGIS Mobile application: GPS averaging and GPS streaming. GPS streaming is only available when collecting the vertices of line and polygon features and cannot be used for the collection of point features.

GPS averaging method

You can use GPS averaging to generate the location of a vertex of a line or polygon or a point feature. The averaging method reads positions from the GPS receiver and derives the most accurate location from the positions read by taking all the positions received that meet the quality filter and calculating an average.

To ensure that enough positions are received to construct an accurate location, you can set a recommended minimum number of positions. Note that the field-worker will be able to override this value, and it is only a recommendation. You can also control how the application will react when the recommended number of positions is reached and/or if the field-worker stops the collection process before meeting the recommended minimum. By default, the averaging method will stop collection of GPS positions when it reaches the recommended minimum. If you uncheck the Automatically Stop option, the averaging process will continue until you tap the Stop button.

If collection is stopped prior to reaching the minimum number, you will receive a message that you did not reach the minimum number of positions recommended for collection of this feature type and be forced to make a decision to either continue receiving positions; accept the location using the current number of positions and calculate an average; or abandon the collection and either start again, change the quality filter values, or choose a different approach to shape collection. If you feel that field-workers do not need to make this decision and trust that they know what they are doing, check the Allow Stop option and they will not be prompted and forced to make this decision.

GPS Averaging settings

GPS streaming method

If the feature layer you are configuring is a line or polygon feature layer, you can also use GPS streaming to collect the shape of a feature. The GPS streaming method will create vertices from positions that are received from your GPS receiver at a defined interval that is specified in the project or can be changed in the field inside the application. The interval is either time or distance based.

The form of movement and/or the shape of the feature you want to collect often dictates whether you will want to use distance or time as the streaming method. For example, you might want to use time as the basis for collection if you are driving a set speed and collecting a street centerline. If you are walking along a path that meanders considerably, you might want to use distance instead.

By default, a distance of 10 meters is chosen. You can change the distance value and unit type from the GPS Streaming settings.

Note:

When using GPS streaming inside the application, you can choose a lateral offset when collecting lines. This allows you to walk or drive alongside the locations that you are collecting rather than having to be directly on top of what you need to collect.

GPS Streaming settings

It is important to note that, like GPS averaging, GPS streaming will only use positions that meet the quality filter you have set when creating vertex locations.