GPS Position Averaging

“I use your Wetland Stakeout app with an Arrow GNSS receiver connected to an IPAD. Is there a way to collect features using position averaging with that app? If no, what APPS do you have that allow for position averaging?”

We have a few apps that provide GPS/GIS data collection functionality with position averaging:

For iPhone only: iGPSGIS II

For iPad only: iCMTGIS II, Utility Data Collection, Farming GPS GIS II/III, Forester GPS GIS II/III

For iPad and iPhone: iCMTGIS III, iCMTGIS PRO

Please let us know what other functionality you are looking for besides point averaging so we might help you identify the app to use. For example, will you want to be able to load a GeoTiff background map in the app? Will you need to export the data as a text file, or just in Shapefiles format? Will you be using a high-precision GPS receiver, such as an EOS Arrow or an iSXBlue II? Or, is a 2 – 5 m position accuracy good enough?


iCMTGIS PRO 1.0.1 released

You might call iCMTGIS PRO the Cadillac of our iOS apps. This app builds on iCMTGIS III and provides Contour – Volume functionality as well as the ability to read the NMEA stream from a number of “External GPS” receivers, such as the Dual XGPS-150A, the Bad Elf and the EOS Arrow.

“External GPS” refers only to those GPS receivers for which the app can parse the NMEA messages. To use any other compatible external GPS, please turn off “Enable the external GPS” check box under GPS setup.

After you have turned on “External GPS” and the GPS receiver is able to compute a position, the app will display a GPS status indicator, such as N3D3, which indicates that the GPS is providing 3-dimensional position and that the PDOP is 3. You will also be able to view the Satellite Status and position errors.

The iCMTGIS PRO and Arrow 200 combo is well suited for doing RTK GPS data collection using the correction data from a RTK network. You will use EOS Tools Pro to connect to the network. With such a setup, the iCMTGIS PRO app will be able to display the RMS error, the age of the GPS differential correction data as well as the differential station ID. This means that Surveyors and GIS field personnel will be able to check the accuracy of the position data collected in real time. In fact, they could also set a limit on RMS such that the system will only record data with errors that fall within the specified limit.

Please also visit the following sites to read our press release:

Using an external GPS with iCMTGIS II on iPad

“For high precision GPS positioning I did connect an external GPS receiver to the iPad via Bluetooth. How and where can I select the GPS receiver in the application? Does it switch automatically between the internal GPS receiver and the external connected via Bluetooth? Where can I see in the application which GPS is connected and how can I decide which one to use and fix as default GPS receiver?”

The iOS devices can only work with GPS receivers approved by Apple. Using an external GPS receiver often improves position accuracy. Assuming the external GPS is an iSXBlue II, a Dual XGPS-150, a Garmin Glo or a Bad Elf GPS receiver, it will provide NMEA data to iOS. The Location Service will distribute the location data to the apps. Currently, all of our apps get the GPS data from the Location Service of the iOS device.

So, to use the external GPS, make sure you have turned on Location Services under Privacy. Also, on the Location Services page, find iCMTGIS II and turn the switch on to make Location Service available for this app. (Do this for the app you are using, such as iGPSGIS II, Utility Data Collection, Stakeout iCMT, Farming GPS GIS, Forester GPS GIS, etc.)

For iOS devices with built-in GPS, how would one know which GPS is being used?

The short answer is: Your iOS device will serve the GPS data from the external GPS when the internal GPS is not actively being used.

Not to leave you in the fog, here is the long answer:

1. To make use of GPS, Location Service must be turned on under Settings and also turned on for the specific app you are using.

2. The iOS device will automatically use the internal GPS if you turn on GPS within an app while no external GPS is connected.

3. When any app is using the internal GPS, iOS cannot work with an external GPS connected to the device.

4. Therefore, to make use of the connected external GPS, you will want to turn off GPS from all running GPS apps (including the one you are actively using) then turn on GPS within the app you are actively using.

5. It will also help to turn on Airplane mode then turn on Bluetooth and make sure Location Service is active for the app.

6. When Bluetooth connection is lost, then iOS uses the internal GPS. This could happen randomly.

Usually, the first time you turn on GPS in the app, the device will connect with the external GPS. So to make sure the device is connected with the external GPS, turn off GPS (from the menu bar of the app such as iCMTGIS II) then turn it on again. Or, reconnect the Bluetooth GPS. The status maybe shown as “Connected”, but just tap on it to make it reconnect again.

How to improve position accuracy of recorded points

When using a consumer-grade GPS, you may find that the device does not pin-point your exact location. Sometimes the GPS marker can be quite a distance away from your actual position.

As we’ve mentioned in a previous post, there are many factors that affect the accuracy of the computed GPS position. Some of those factors are out of your control. GPS differential correction can help remove some sources of position errors, but consumer-grade GPS does not provide differentially corrected GPS data.

Nevertheless, you can do a few things to help improve the position accuracy of the points logged. First of all, make sure your device has a  good view of the sky and is not obstructed by nearby objects (especially reflective ones) or buildings. Secondly, after turning on the GPS device, wait 8 – 10 minutes until it settles down. This will be indicated by the GPS marker  settling close to where it should be (if you are doing this at a reference location) and is not jumping all around. Then you would start recording your point positions. If you are using iCMTGIS II or iGPS GIS II, you can usually get a reasonably good position by setting the Time Session to 300 seconds (5 minutes).

Remember that each time you turn GPS off from your app and turn it back on, you will need to wait a few minutes for the GPS to settle down. Same if you turn off an external GPS device then turn it on using its ON button.