Key takeaways
- A phone or tablet works fine with a TerraClear Rock Map. Under open sky, it gets you within about 2 to 6 m (6 to 20 ft) of each stone.
- A dedicated GPS receiver gets you closer, to about 30 cm (1 ft), and updates your position up to 10 times a second.
- RTK gets you closest. With corrections, the receiver is accurate to about 2.5 cm (1″), and the remaining error is mostly the map’s.
- What you see in the field combines two errors: the map’s placement, generally under 1 m (about 3 ft), and your device’s GPS. The receiver you choose controls the second one.
On this page
- How accurate is GPS for stone picking?
- Where does in-field accuracy come from?
- Is a phone or tablet good enough?
- Why does a dedicated receiver do better?
- Which GPS receiver should you use?
- Why is RTK the best option?
- Where can you get RTK corrections?
- What else affects GPS accuracy?
- Do you need cell service?
- How do you choose the right setup?
- Frequently asked questions
How accurate is GPS for stone picking with a TerraClear Rock Map?
A phone used as GPS for stone picking typically gets you within about 2 to 6 m (6 to 20 ft) of each stone. A dedicated receiver can get you as close as 30 cm (12″), and RTK within about 2.5 cm (1″). The TerraClear Rock Map marks where each stone sits in the field. The GPS in your phone, tablet, or receiver marks where you are. The closer those two agree, the faster you pick.
Manufacturer specs and published research put the three options roughly here:
| Tier | Good: phone or tablet | Better: dedicated receiver | Best: RTK |
|---|---|---|---|
| Typical accuracy | About 2 to 6 m (6 to 20 ft) | As good as 30 cm (12″) (varies by location) | About 2.5 cm (1″) or better |
| Position updates | Usually once per second (1Hz) | Up to 10 to 20 per second | Up to 10 to 20 per second |
| Extra hardware | None | Receiver, plus mount or cable | Receiver plus a correction source |
| Best fit | Light stubble, scattered stone | Most fields | Dense stone, heavy residue, any condition |
Treat these as typical ranges, not guarantees. Your own numbers will move with shelterbelts, terrain, and the location the day you pick.
Where does in-field accuracy come from?
In-field accuracy is two errors added together: the map’s placement margin, generally under 1 m, and your device’s GPS error.
Start with the map. TerraClear flies your field by drone and captures imagery. AI picks out stones 20 cm (8″) and larger and tags each one with a precise GPS location. Capturing and stitching that imagery adds a small margin of its own, generally under 1 m (about 3 ft), depending on the sensor and terrain. The fall stone picking guide covers how TerraClear flies your fields in more detail.
Then there’s your device. Its GPS fix is the part you control, and it swings the most. A phone can miss by 5 m or more. An RTK receiver is repeatable with sub-inch accuracy.
The two add up to what you see from the cab. On a phone, the device is most of the error. On RTK the device error shrinks to almost nothing, and what’s left is the map’s margin, which is small enough that a 20 cm stone should be in plain view as you pull up.
Is a phone or tablet good enough for a TerraClear Rock Map?
In a lot of fields, yes: a phone gets you within about 2 to 6 m, which is plenty on clean ground with scattered stone. Most farmers start with the phone or tablet they already carry. The TerraClear FAQ is direct about the trade-off: accuracy in the field “is dependent upon your device.”
GPS.gov says smartphones are typically accurate to within a 4.9 m (16 ft) radius under open sky, with buildings, bridges, and trees making it worse (GPS.gov). A 2024 Cal Poly test of 13 phones averaged about 3.2 m (10.6 ft) of error, with newer dual-frequency phones doing a little better than single-frequency models (Cal Poly). Closer to home, Olds College in Alberta has a GNSS accuracy primer on why receivers and corrections matter on the Prairies, including how far positions drift without good corrections.
On clean ground, the phone puts you in the right area and you spot the stone. Heavy canola or cereal stubble and dense patches change that. If you’re 5 m off and three stones sit inside the circle, you spend the time hunting instead of picking.
Phones also update slowly, usually once a second, and some models lag when cell service is weak.
[Image: terraclear-rock-map-phone-gps-connected-ca.webp. Alt: TerraClear Mobile on a phone showing a Rock Map with GPS connected and a suggested stone picking route. Caption: TerraClear Mobile showing GPS status and a suggested pick route on a phone.]
Why does a dedicated GPS receiver do better?
It has a better antenna and chip than a phone. The receiver pairs with your phone or tablet over Bluetooth or a cable, and the app uses its position instead of the built-in one. Most receivers also pull free satellite corrections (WAAS, which covers the farmed Prairies) to tighten the fix.
Receivers update faster, too. At common picking speeds, this is how far you move between position updates:
| Speed | Gap at 1Hz (most phones) | Gap at 10Hz (receiver) |
|---|---|---|
| 5 km/h | 1.4 m (4.6 ft) | 14 cm (5″) |
| 8 km/h | 2.2 m (7.3 ft) | 22 cm (9″) |
| 13 km/h | 3.6 m (11.8 ft) | 36 cm (14″) |
Update rate has nothing to do with how accurate each fix is. What it changes is how well the blue dot keeps pace with the machine. At 10Hz the map turns when you turn, so you aren’t waiting on it as you swing toward the next stone.
Which GPS receiver should you use with your Rock Map?
Three receivers work with the TerraClear Mobile app. Any of them beats a phone, and the right one depends mostly on whether you run Android or iOS and what’s already on your equipment. They’re listed from the best all-around fit to the most situational.
First, update TerraClear Mobile to version 3.4.0 or later. That release added support for external receivers.
1. John Deere StarFire receiver
A StarFire already on your tractor, air drill, or sprayer may be the best receiver you could use. TerraClear requires an SF2 or higher correction signal. John Deere dealer Van Wall lists SF2 at plus or minus 5 cm (2″) pass to pass, and SF3, SF-RTK, and RTK are tighter (Van Wall). The position holds steady without lag, and it keeps up with quick direction changes in dense stone.
It only works with Android, not iPhone or iPad. It also takes some setup: a mounting spot, power, and a stand-alone harness with RS232 output, such as the Ag Express AE3070P, which is listed for StarFire 3000 and 6000. Add an RS232-to-USB-C adapter and a USB-C splitter so the tablet charges while it reads the receiver. On a StarFire 7000 or 7500, check harness fit with TerraClear support or your dealer.
2. Columbus P-70 Ultra
The Columbus P-70 Ultra delivers high-quality GPS with no fees or subscriptions, and of the three, it’s the easiest to buy in Canadian dollars, through CanadaGPS.ca. It’s a small dual-band receiver on a magnetic antenna, so moving it from the half-ton to the loader takes seconds. The manufacturer lists about 30 cm (1 ft) accuracy with free satellite corrections and update rates up to 20Hz. For picking, 10Hz is the smoothest setting.
The manufacturer lists it for Android 10 or later. It has no internal battery and runs on USB power, so you’ll need a power source in the cab. Connecting it to your Rock Map takes a few minutes; the P-70 Ultra setup video walks through it.
3. Juniper Systems Geode
For iPhone and iPad users, the Juniper Geode is the one to look at. It works with both iOS and Android, runs all day on its battery, and mounts with a magnet. Juniper lists about 30 cm (1 ft) accuracy with free satellite corrections. In Canada, resellers such as Canal Geomatics carry it.
Out of the box, the Geode updates at 1Hz. The 10Hz and 20Hz rates and multi-frequency tracking are paid upgrades, and 10Hz makes a real difference while picking, so ask Juniper or your reseller which options you need before you buy. Juniper sells single-frequency and multi-frequency models, and the multi-frequency one is built for RTK.
Why is RTK the best GPS option for picking stones?
RTK (real-time kinematic) checks your receiver’s position against a base station at a known location and corrects it, down to about 2.5 cm. Olds College notes that radio RTK can reach sub-inch accuracy close to the base, with radio range usually 8 to 16 km (5 to 10 miles) over open ground (Olds College).
With RTK, your device stops being the weak link. What you see in the cab is close to the map’s own placement. RTK gives the best experience in every field condition, and you’ll notice it most in heavy residue and dense stone.
Each of the three receivers can run RTK. A StarFire uses SF-RTK, which comes by satellite with no base station, or RTK Radio. Deere lists those at plus or minus 2.5 cm (1″) pass to pass. Mobile RTK over cellular has limited availability in Canada, so ask your John Deere dealer what’s offered where you farm. The P-70 Ultra has an RTK mode that takes corrections streamed over the internet (a method called NTRIP) through your phone or tablet’s data connection. The manufacturer lists it at 1 cm plus 1 part per million of the distance to the base, which is under an inch near the base. The Geode does the same over NTRIP once you add its RTK upgrade.
Where can you get RTK corrections in Western Canada?
Western Canada has no free provincial RTK network like the ones some US states run. Farmers here use subscription networks, dealer networks, or satellite corrections.
- Can-Net is a subscription RTK network with more than 300 stations across Canada, including broad coverage of farmed Prairie land.
- LATNET covers Alberta and B.C. and works with major equipment brands.
- Many Prairie dealers run shared RTK bases for customers. Ask yours what covers your RM.
- Deere’s SF-RTK and similar satellite services need no base station and no cell signal.
Network RTK needs cell service in the field, and Olds College points out that coverage can be a real issue in parts of Western Canada. On a quarter with spotty service, satellite-delivered corrections don’t depend on it.
What else affects GPS accuracy in the field?
Antenna placement, shelterbelts, space weather, and phone lag can all degrade your fix, even with a good receiver. The cab itself blocks signal, so a receiver sitting on the dash does worse than one up top. University of Tennessee Extension recommends mounting the antenna on top of the machine along its centerline (UT Extension). Near shelterbelts and yard sites, signals bounce off trees and buildings, and the fix gets worse along field edges.
Space weather can throw you off too. The May 2024 geomagnetic storm disrupted GPS guidance across North America, RTK systems included (Agriculture Dive). If your fix looks wrong for no obvious reason, check Space Weather Canada.
On a phone, a weak cell signal or an older device can also add lag.
Do you need cell service to use a Rock Map in the field?
Not for position, if you run an external receiver. A receiver like the P-70 Ultra supplies the GPS fix, so you don’t need a cellular tablet or a hotspot. Without one, your tablet needs its own GPS chip. Cellular-capable tablets have one and many Wi-Fi-only tablets don’t, but even then you don’t need a hotspot just for GPS.
Syncing is different. With cell service or a hotspot, TerraClear Mobile uploads your traveled path, picked stones, and any tags you add every five minutes, so others in your operation can follow along. If nobody needs that live, sync on Wi-Fi at the end of the day.
[Image: terraclear-rock-map-tablet-gps-connected-ca.webp. Alt: TerraClear Mobile on a tablet showing a Rock Map, GPS connection status, and traveled path. Caption: Your traveled path and picked stones sync to the cloud every five minutes when you have a data connection.]
How do you choose the right GPS setup for your farm?
The right GPS for stone picking depends on your fields and the device you carry:
- Light stubble, scattered stone, any phone: start with the phone or tablet you have.
- Most fields, Android: use a StarFire you already own, or a P-70 Ultra.
- Most fields, iPhone or iPad: use a Juniper Geode with the 10Hz upgrade, or a P-70 Ultra.
- Heavy residue, dense stone, or the best results everywhere: add RTK to any of the above.
The Rock Map does the finding no matter which you choose. Farmers using TerraClear Rock Maps find about 3x more stones and pick about 3x faster, and better GPS cuts the time spent hunting for each one.
Not sure which setup fits your operation? Talk to a TerraClear rep or call Customer Support at 425-274-2905 (option 2). The TerraClear FAQ covers more.
Frequently asked questions
No. The TerraClear Rock Map finds the stones. A better receiver gets you to each one faster, so you spend less time searching.
The manufacturer lists the P-70 Ultra for Android. For an iPhone or iPad, the Juniper Geode works with iOS.
Yes, with an Android tablet and the right harness. You’ll need power, a stand-alone RS232 cable, an RS232-to-USB-C adapter, and a USB-C splitter.
Use 10Hz if your receiver supports it. At 8 km/h, a 1Hz fix updates every 2.2 m and a 10Hz fix about every 22 cm.
If you pick dense stone or work through heavy residue, yes. RTK cuts your device error from metres to about 2.5 cm, which leaves mostly the map’s own placement.
Version 3.4.0 or later.
RTK makes your position very accurate, but the map’s placement has its own small margin, generally under 1 m. Together they still put you close enough to see a 20 cm stone as you approach.
Read Fall Stone Picking: Map Stones After Harvest or visit the Rock Management page.