# External GPS

External GPS receivers can be connected to your mobile device via Bluetooth and used in Mergin Maps mobile app to achieve higher GPS accuracy.

There are several extra position variables that can be useful to record when doing the survey with external GPS, such as the GPS antenna height, GPS device name as well as metrics like horizontal, vertical or position dilution of precision (HDOP, VDOP, PDOP).

Note that external GPS devices usually return orthometric heights (ellipsoid with the geoid separation applied).

Before you start:

  • Set up your device according to the instructions of its manufacturer. You should continue only when you are sure that the device is working and sending data.
  • Make sure that your mobile device offers the functionality to pair with an external GPS device and that it communicates through a standard interface.

GPS and GNSS terminology

The term GPS, which stands for the Global Positioning System, is used in Mergin Maps mobile app and in this documentation in a broad sense as a synonym of the global navigation satellite systems (GNSS). We are aware that it is not exactly correct, but GPS is commonly used and understood in this context.
If you have a device that can receive signals from other GNSS (such as BeiDou, Galileo, GLONASS), Mergin Maps mobile app can use them as well.

# GPS antenna height

External GPS antenna is often used on a surveying pole. To obtain the correct ground elevation, it is necessary to subtract the height of GPS sensor above the ground from the measured elevation.

The height of the GPS antenna can be set in Mergin Maps mobile app Settings. If GPS antenna height is set to a non-zero value, it is also displayed next to the GPS accuracy.

GPS antenna height setup


GPS antenna height can be recorded during the survey by using the extra position variable @position_gps_antenna_height as a default value.

# External GPS on Android

In Android, there are two ways how to use an external GPS device in Mergin Maps mobile app:

  • directly through the app as an external receiver
  • connecting GPS receiver to your mobile device using an app that will provide mock location

It is strongly recommended to use the direct connection in Mergin Maps mobile app. It is easier, quicker, provides more data, such as HDOP and fix quality. The app will report which device is used and take care of reconnecting in case of lost connection. Using mock location should be used only when it is not possible to connect the GPS receiver directly via Mergin Maps mobile app.

  1. Connect the GPS receiver to your mobile device via Bluetooth.
  2. Open Mergin Maps mobile app and navigate Select GPS receiver in Settings:
    Select GPS receiver
  3. Tap on Connect new receiver in GPS receivers panel:
    Connect new GPS receiver
  4. Choose your GPS receiver from the list of Bluetooth devices. You might be asked to pair your device during this process.
    Connect to bluetooth GPS device
  5. You can now switch between internal and external receivers:
    Available GPS receivers

# How to connect external GPS receiver in Android via mock location


Mock location should be only used if you are unable to connect the external GPS directly in Mergin Maps mobile app.

External GPS can be connected and configured in Android to provide mock location using GPS apps (e.g. Bluetooth GPS (opens new window) or apps from specific GPS manufacturers) as a source of GPS signal. Mergin Maps mobile app and other apps in your device will get the GPS position from the external GPS.


GPS manufacturer's apps often provide a setup for a NTRIP client, through which you are able to receive GPS corrections and achieve centimetre level accuracy.

To enable Allow mock locations in Android:

  • enable Developer options, which are hidden by default:
    • On newer Android versions, go to Android Settings > About phone and scroll down to find the Build number. Tap the build number 7 times. A message appears with a count-down until you become a developer.
    • On older Android versions, the developer settings can be made visible under the Android Settings > Applications > Development.
  • Once you have the developers option enabled, go to System > Advanced > Developer options > Select mock location app and select your GPS app.

The mock location will be used by Mergin Maps mobile app automatically as if received from the internal receiver.

# External GPS on iOS

Direct connection via Mergin Maps mobile app is not possible on iOS devices. External GPS can be connected to your iOS device via Bluetooth. Depending on the GPS manufacturer, there may be a custom app, which then provides position to your iOS device. Mergin Maps mobile app sees this as an internal GPS receiver automatically, without additional configuration.

iOS GPS receiver

# External GPS troubleshooting

If you encounter issues with external GPS in Mergin Maps mobile app, make sure that:

  • your Bluetooth is turned on
  • GPS is paired with your mobile device
  • your GPS receiver is turned on, has battery and is sending data in NMEA format, which is readable by Mergin Maps mobile app
  • remove the device in Mergin Maps mobile app, and add it again from Mergin Maps mobile app

# List of known supported GPS devices

External GPS functionality depends on the manufacturer and on the specific model of your GPS device. These devices are currently known to work well:

Manufacturer Model Android iOS
Carlson Carlson Brx71 yes (mock location) unknown
Carlson Carlson xML2 1 yes (mock location) unknown
Emlid Emlid Reach RX2 yes no
Emlid Emlid Reach RS+2 yes no
Emlid Emlid Reach RS2/RS2+2 yes no
Emlid Emlid Reach RS32 yes no
Garmin GLO 2 yes yes
Juniper Systems Geode GNS33 yes yes
Leica Leica FLX1004 yes (mock location) no
Leica Leica FLX100 plus4 yes (mock location) yes**
Leica Leica Zeno GG04plus4 yes (mock location) yes**
marXact UNI-GR1 yes no
marXact UNI-GR2 yes no
Trimble Trimble Catalyst5 yes (mock location) unknown
Trimble Trimble R15 yes (mock location) unknown
Trimble Trimble R25 yes (mock location) unknown
proNIVO PNR216 yes (mock location) no
SingularXYZ P17 yes unknown
Bad Elf GPS Pro yes yes
Bad Elf GPS Pro+ yes yes
Bad Elf GNSS Surveyor yes yes
Bad Elf Flex Mini (standard or extreme) yes yes
Bad Elf Flex (standard or extreme) yes yes
  • 1: Carlson Brx7, Carlson xML2 - through Carlson Layout (opens new window) which will set a mock location in Android.
  • 2: Emlid Reach RX, Emlid Reach RS+, Emlid Reach RS2/RS2+, Emlid Reach RS3 - directly via Bluetooth connection, has an internal NTRIP client to receive corrections. Possible to set a mock location and connect the receiver via Bluetooth using GPS Connector (opens new window) or WiFi using Lebefure NTRIP Client (opens new window). Android only.
  • 3: Geode GNS3 - through Geode Connect app on Android (opens new window) or iOS (opens new window), which also acts as an NTRIP client and sends corrections to the device.
  • 4: Leica FLX100, Leica FLX100 plus, Leica Zeno GG04plus - through Leica Zeno Connect app on Android (opens new window) which also acts as a NTRIP client and sends the corrections to the device. The app will set a mock location in Android. It is also possible to connect directly via Bluetooth (even multiple phones can be connected at once), but if no phone has Zeno app running, there will be no corrections available.
    ** Leica Zeno Connect is also available on iOS (opens new window). It is known to support Leica FLX100 plus and Leica Zeno GG04plus. However, on iOS, the vertical accuracy information is not transferred to Mergin Maps mobile app through Leica Zeno Connect. Mergin Maps mobile app will not display the correct value of the vertical accuracy.
  • 5: Trimble R1, Trimble R2, Trimble Catalyst - through Trimble Mobile Manager app (opens new window) which also acts as a NTRIP client and sends the corrections to the device. The app will set a mock location in Android.
  • 6: proNIVO PNR21 - through Attenberger Connector app on Android (opens new window).
  • 7: SingularXYZ P1 - the device uses a SIM Card that can be configured for NTRIP. It can be connected to Mergin Maps mobile app via Bluetooth (without using a mock location).

Did you use a GPS that is not in this list? Share your experiences with us! (opens new window)