Configuring GPS collection settings
When your field workers use the GPS functionality in field applications to collect shapes of features, they need to be aware of the quality of the GPS positions received. When you author a project, it is recommended that you identify the optimum time for the field workers to collect GPS positions based on position dilution of precision (PDOP). There are some existing free applications and online tools for this purpose:
- Trimble Planning: http://www.trimble.com/planningsoftware_ts.asp?Nav=Collection-8425
- Web Mission Planning: http://www.ashtech.com/web-mission-planning-2712.kjsp
Depending on the field conditions, the time of day, and a few other variables, your field workers might still have problems using their GPS receivers because it may be hard to completely plan before going out into the field. Under such circumstances, it is important to fine-tune the GPS collection settings to help ensure high data quality.
GPS data collection in ArcGIS for Windows Mobile uses the concept of a quality filter to get the best accuracy from a GPS receiver. The GPS quality filter is the GPS data quality settings for your mobile project (available in Mobile Project Center under Project > GPS Collection Settings). If GPS data doesn't meet quality filter criteria, the GPS location won't be used for vertex collection or editing will be ignored. If you want field workers to change the Fix Type and Maximum PDOP settings in the field, check the Allow client to change setting. Quality filtering is based on two key factors: GPS fix type and PDOP.
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GPS Collection Settings dialog box in Mobile Project Center |
GPS fix type
Depending on the GPS receiver in use and the type of GPS data collection your field workers perform, you may want to filter the positions received from the GPS receiver based on a certain fix type. For example, you can ensure that only differentially corrected GPS locations are used, while ignoring all autonomous GPS positions, when using a WAAS-enabled receiver.
If the GPS fix type from a GPS receiver is different from the GPS fix type that you specified in the quality filter, your field workers may still be able to collect GPS positions. This can occur if the fix type they're receiving is of higher quality than the one specified in the filter. The hierarchy of fix types is as follows (from least accurate to most accurate):
- Estimated
- GPS fix
- Differential
- Float real-time kinematic
- Real-time kinematic
For example, if the GPS receiver fix type is differential GPS (DGPS) fix and the filter setting is GPS fix, the field workers are still able to use DGPS positions. But if the filter setting is DGPS, they cannot use GPS fix for data collection.
PPS fix is a military secured fix type. Once it's set to PPS fix, PPS fix will be the only fix type for data collection.
PDOP
In addition to the type of fix that the field workers receive, you can also filter positions based on the geometric strength of the GPS satellite configuration, which is represented by PDOP. Setting a PDOP value as part of the GPS quality filter ensures that only positions with a PDOP equal to or less than the set value will be used to create the feature vertices. If the devices are GPS+GLONASS capable, they have better PDOP compared to GPS alone. Here are the meanings of PDOP value:
- 1: Ideal
- 1–2: Excellent
- 2–5: Good
- 5–10: Moderate
- 10–20: Fair
- >20: Poor