The original version of this document is located at https://docs.qfield.org/how-to/gnss

Positioning (GNSS)

QField can make use of the internal GNSS (Global Navigation Satellite
System, like GPS, GLONASS, Galileo or Beidou). QField can also connect
to external antennas through NMEA streams over Bluetooth, TCP, or UDP
connection.

GNSS devices are also capable of measuring the altitude next to the current 2D
position on the earth surface.

Visualization

When positioning is activated, your position will be shown in blue on the map.
Your location is visible either as a blue dot if you are still or as an arrow indicating your movement direction
if you are moving.

The blue beam indicates the current orientation of your device if the device has
a builtin magnetic compass.

A circle around your current position indicates the precision as reported by the
positioning device.

Configuration

The following settings are available in QField settings’ positioning tab.

Measure (M) value

When digitizing a geometry onto a vector layer that contains an M dimension,
QField will add a measurement value to individual vertices whenever the
coordinate cursor is locked to the current position.

By default, the value will represent the captured position’s timestamp (milliseconds
since epoch). You can change this value using the combo box in the settings’
positioning tab.

The available values to chose from are timestamp, ground speed, bearing, horizontal
accuracy and vertical accuracy as well as PDOP, HDOP and VDOP.

Accuracy requirement

A minimum desired accuracy for measurements can be defined. The quality
will be reported in three classes, bad (red), ok (yellow) and excellent
(green). These colors will show up as a dot on top of the GNSS button.

The thresholds can be defined in the settings’ positioning tab.

If the Enable accuracy requirement setting is activated, you will not
be able to collect new measurements with the coordinate cursor locked to
the current position with an accuracy value which is bad (red).

Antenna height compensation

The height of the antenna pole in use can be defined in the settings.
Any measured altitude will be corrected by this value.

Altitude correction / vertical grid shift

Altitude values can be corrected with vertical grid shift files to
calculate orthometric height.

Vertical grid shift files have to be made available to QField by putting
them into the QField app folder <drive>:/Android/data/ch.opengis.qfield/files/QField/proj.

Once the grid shift file is placed there, it is available in QField in
the Positioning settings under Vertical grid shift in use.

If you are using altitude correction and an external positioning device
is used, consider turning Use orthometric altitude from device off.

The formats currently supported are:

• GeoTIFF (.tif, .tiff)
• NOAA Vertical Datum (.gtx)
• NTv2 Datum Grid Shift (.gsb)
• Natural Resources Canada’s Geoid (.byn)

For example:
For the transformation from ETRS89 (reference ellipsoid GPS) to NAP (Dutch) users can download
the file nlgeo2018.gtx from NSGI and put it in the directory.

To obtain precise altitude data for Cadastral Surveying in Switzerland, users can access the file correction of the vertical grid shift through Geoid OGD from Swisstopo.
Following the download, users are advised to perform a conversion of the file labeled chgeo04_htrans_lv95.agr to chgeo04_htrans_lv95.gtx.
The QGIS processing algorithm gdal:translate (convert format) can be used for that.

Usage

📱 Fieldwork

A short press on the GNSS button will turn on the GNSS and center to the
current location once positioning information is available.

Activate edit mode and press on the target button, the cross in the
center means it is using GNSS positioning.

A long press on the GNSS button will show the positioning menu.

Inside the positioning menu you can turn on the Show position information
which will show the current coordinates which are reprojected into the
project CRS along with precision information.

Using an external GNSS Receiver

📱 Fieldwork

QField supports connecting to external GNSS positioning devices via NMEA streams through Bluetooth, TCP,
or UDP connections.

In Settings > Positioning, you can find a set of buttons to add, edit, or delete external
devices as well as a dropdown list to switch between internal and saved external GNSS devices.

The breakdown of connections support by platform is as follow:

(*) Bluetooth support on Windows occurs through the virtual serial port automatically
created by the operating system when it connects to the GNSS device.

The NMEA sentences currently supported are GGA, RMC, GSA, GSV, GST, VTG, HDG and HDT.

External receiver log

In Settings > Positioning if you have selected an external receiver as the positioning device, you will find a switch Log NMEA sentences from device to file. If this is activated, all NMEA sentences coming from external positioning devices will be logged to a file.

The logs will be placed in Android/data/ch.opengis.qfield/files/QField/logs.

Mock location

📱 Fieldwork

It is possible to provide a mock location via a separate android app to
QField. There are several options for this, one of them is Android NTRIP Client.

To use this you have to enable mock locations on your Android device.

Averaged positioning functionality

📱 Fieldwork

There is a function that allows you to digitize using averaged positions.

The survey will start by pressing and holding the add vertex button, which will start collecting positions.

While collecting, an indicator will appear on top of the coordinate cursor showing a text reflecting the current number of collected positions.
If an averaged position minimum count requirement is active, a progress bar will also be present indicating the progress towards meeting that requirement.

The setting to activate an average position minimum count threshold can be found in QField settings’s positioning panel.
When active, holding the add vertex button is not required, a short tap on the button will begin the collection of positions and automatically add the averaged position when the minimum count requirement is met.

When using @gnss_* or @position_ variables on averaged positions, the variable will also represent the average over all collected samples.

Project configuration

🖥️ Desktop preparation

Positioning variables

You can get access to positioning information through additional
expression variables accessible in the attribute form. These will
only be available when positioning is enabled.

These variables are commonly used as part of default values expressions
for fields to keep track of the quality of individual measured points.

A common use case is recording the horizontal accuracy, which can be done by using the variable @position_horizontal_accuracy.
Another often used strategy is using the altitude of the current measurement which can be achieved with z(@position_coordinate).
For a complete listing of all available variables, refer to the expression variables reference documentation.

Information for GNSS Z value with Vertical grid shift in use:
- Antenna height compensation=False

Vertex logger

It is possible to setup a log layer of the collected vertices. This
allows to keep track of meta data for each vertex like GNSS quality
attributes and more. To set this up, a point layer can be added to the
project and attributes configured to store this information.

{width=”600px”}

Then you should assign the role digitizing logger to a point layer.

Go to QFieldSync > Project Properties

{width=”600px”}

To be most effective, the layer attributes should have default values that
relies on the positioning variables enumerated above.