Phone GPS or a receiver
What the phone's GPS and an external GNSS receiver each give you, how the app picks between them, and when you need a receiver.
On this page
OpenSurvey3D works with the phone alone, and it works better with an external GNSS receiver. This guide explains the difference, how the app decides which one to use, and how to tell which one you're getting.
What each source gives you
| Built-in GPS | External GNSS receiver | |
|---|---|---|
| Typical horizontal accuracy | A few metres | Centimetres with an RTK fixed solution. A metre or two without corrections |
| Heights | Can be 5–15 m off | Centimetre-level with RTK, reduced by your antenna height |
| Fix type | Recorded as a phone GPS fix | Fixed, float, DGPS, single and so on, as the receiver reports it |
| Recorded detail | Position and the accuracy iOS reports | Satellites, DOPs, the receiver's error estimates, correction age and source, antenna height, and which receiver (with serial number when it reports one) |
| Raw record | None | The raw NMEA stream, kept with the project. See Keep raw NMEA logs |
| Needs | Location permission | A receiver on Bluetooth or Wi-Fi. Corrections for RTK |
The phone is fine for mapping assets you'll find again by eye: signs, trees, valves, rough boundaries. For property corners, stakeout, design work or anything where centimetres matter, you need a receiver with RTK corrections.
How the app picks the source
You choose a source in Precision → Source (hold the accuracy pill to open Precision). The Positioning source screen lists:
- Built-in GPS: "Phone location · a few meters".
- Bluetooth receiver: see Connect a Bluetooth receiver.
- Wi-Fi receiver (TCP): see Connect a Wi-Fi receiver.
Once you've connected a receiver, the app uses it whenever it's delivering fixes, and falls back to the phone's GPS automatically when it isn't:
- If the receiver has no fix, or hasn't sent a positioned fix in the last 5 seconds, positions come from the phone.
- If the connection drops, the pill shows RECONNECTING and positions come from the phone until the receiver is back.
To stop using the receiver, tap Built-in GPS on the Positioning source screen.
An averaged occupation sticks with the source it started on. If a receiver drops partway through an average, phone positions aren't mixed into it.
How the source is labelled
The Precision sheet's Source row shows the source and a line of detail. The Precision & receiver row in Settings shows a short status too.
| Source label | Meaning |
|---|---|
| Built-in GPS | The phone's own GPS. Detail: Phone location |
| Name (Bluetooth) | A Bluetooth receiver connected in the app |
| Receiver (NMEA TCP) | A Wi-Fi receiver. The Source row shows its address, for example 192.168.42.1:9001 |
| External accessory via iOS Location Services | See below |
"External accessory via iOS Location Services"
Some Bluetooth receivers (Bad Elf, for example) are location accessories to iOS. Once paired, iOS passes their positions to every app as if they were the phone's location, even if OpenSurvey3D has never connected to the receiver.
The app detects this and labels those positions External accessory via iOS Location Services. The Precision sheet's Source detail reads "Relayed by iOS — connect the receiver above to record fix type and accuracy".
Those positions can be very good, but the app gets no fix type, no receiver error estimates and no correction age, so none of the evidence a survey needs is recorded. Their fix type is recorded as unknown. An accuracy gate flags them but never blocks them, because it can't judge them.
Location permission
The phone's GPS needs location permission. An external receiver works without it: its positions come straight from the receiver. You still want permission granted for the map's blue dot when the receiver isn't connected. See Install and permissions.