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Introduction: Why Do We Need “Differential” GPS?
Suppose you want to accurately measure the boundary of a plot or field. You might assume getting the GPS coordinates of the corners is sufficient—after all, any software can calculate area or perimeter from those. But here’s the problem:
Unfortunately even the best of standard GPS on phones or handheld devices are accurate only to about ±5 meters (95% of the time). This means each point you record could actually be anywhere inside a 5-meter-radius circle. If every corner point is off by a few meters, your drawn plot could shift, expand, or shrink, leading to an area calculation off by hundreds of square meters!
Why Does a GPS Have this Error?
While there could be various sources of error, but they can be clubbed broadly in two categories:
Random:
Multipath: GPS signals can bounce off nearby buildings, trees, or the ground before reaching your device. This confuses the GPS receiver and leads to inaccuracies, especially in cities or forests.
Receiver noise: Minor inaccuracies from electronic noise, hardware quality, or your antenna placement can also affect your position, usually by about a meter or less.
Systematic
Satellite clock & orbit uncertainty: The clocks on satellites and their published orbital data aren't perfect, leading to small timing and position errors.
Atmospheric refraction: GPS signals are slightly delayed as they pass through the ionosphere and troposphere, altering their travel time and causing several meters of error.
How to reduce/eliminate these errors
Random: Multipath and receiver noise are unique to each device and local environment. Multipaths can be avoided by keeping the mobile open to the sky and devoid of high buildings. The noise being totally random can be minimised by time averaging.
Systematic: Satellite and atmospheric errors affect everyone in the same region receiving signals from the same set of 4 satellites ,at a given moment. Unfortunately it is not possible to predict this error by any mathematical modelling . At the same time, fortunately, since this error is the same for a region at a given point, if we calculate an error at one place and apply it at the other in the same region, there is a method of removing this error also.
How Do Survey Professionals Fix This?Professionals use a method called Differential GPS (DGPS) or RTK (Real-Time Kinematic). Here’s how it works:
A reference receiver (the "Base") is placed on a precisely known spot. Every second, it compares its current GPS value with the Correct GPS , calculating the error (known as an "error vector"). The Base then broadcasts this live error information via radio or internet to all "Rover" receivers in the area. Rovers correct their own readings using this information. Because most errors are shared spatially, each Rover’s location error shrinks from 5 meters to just a few centimeters!
However, this professional setup requires expensive hardware, radio links or subscriptions, and significant technical skill.
Note: GNSS means Global Navigation Satellite System
What Makes This Web App Different?
While the underlying DGPS principle is the same, this application uses a simplified yet patented approach:
Systematic Error Correction
GPS errors that are regional or systemic are calculated at the Base Station.
These corrections are shared in real time with Rover devices, which apply them instantly.
Random Error Reduction
GPS errors that are random or local in nature are minimized through a patented algorithm.
The system takes 30 readings at a point, removes the top and bottom 10% outliers, and applies the TRIMMEAN function for maximum accuracy.
Legal Notice :Both of the above approaches, implemented solely through smartphone applications, are protected under a registered patent owned by the creator of this application. Any unauthorized use, replication, or adaptation will be subject to legal action.
In addition this app also eliminates the possibility of gross error or human errors by eliminating data noting, data transferring, applying settings etc. Ready to use KMLs are created on the fly on the mobile itself which eliminates the possibility of human errors also.
Any restriction on the type or model of phone:
This web app will work on any smart phone and will give better results compared to any other application or single-band-GNSS handheld GPS.
But for survey grade results ( error< 10 cm) it is important that both base and rover should have Dual Band GNSS capability and be preferably of the same model. Check list of Dual Band GNSS mobiles recommended for this app.
Dual Band GNSS mobile is important as it offers much better accuracy than a single band device.
The same model is important to ensure the error due to different hardware and internal software can also be negated. Very close models having the same GPS system ( Dual GNSS + same satellite sources) should also work equally well.
In addition, it has following important user-friendly features making accurate survey possible even by a non-technical person:
KML export for Google Earth or GIS software.
Live map and labeled points.
Only two phones and this web-app are needed—no technical expertise required!
Result: Super-accurate mapping (down to centimeters or less than a meter) at a fraction of the cost—and as easy to use as Google Maps. In a nutshell this app gives survey grade accuracy with unmatched convenience and speed.
App Walkthrough
Base station mode
Use this if you know your Base location accurately. If not, use "Calibration mode” to get the GPS locations to the desired accuracy. At the opening it will put current GPS values as default. 30 sec TRIMMEAN GPS coordinates may also be filled. These can be edited if needed.
For area length measurement work: The default GPS value for the base station is sufficient.
For boundary marking: Use accurate GPS coordinates up to 7 decimal places.
Important: The Base phone must be open to sky and stationary during logging.
The userID is the base ID which will to be used in the Rover for fetching error correction.
To start: Press “Start Logging”. The phone logs GPS for 30 seconds, takes TRIMEAN and then repeats every minute.
2: Rover Unit
Use your second phone for this.
If some base unit is active on your UserID it will automatically appear else it will show "Base station not active".
Click "Capture point" to start capturing location. The app reads the Rover’s GPS for 30 sec, takes TRIMMEAN, fetches error correction and applies it to get the correct GPS value.
In "Base station not active" situation, the error correction will be nil but still 30 sec time averaging will be done, which will also give 1-2 m accuracy.
Note: For best results the rover unit should be stationary during and for at least 30 sec before “Capture point”.
Other useful features:
Live map
Waypoint list
Export KML (lines, polygons, points)
3: Calibration mode
Use when you don’t know your Base’s coordinate in advance. Place the phone outdoors with a clear sky and steady internet/power.
It displays following:
Start time
Number of readings
Average latitude & longitude
Spread (variance) in values
Let it run for 24 hours for best accuracy. Use the calculated average later as your trusted Base location. All these points, along with timestamps, can also be downloaded in a CSV file and used for any kind of analysis.
Accuracy Tips
Modern phone GPS with Dual band GNSS capabilities are as good as any standalone devices—with the right care!
Place the Base away from trees and walls.
Once logging starts, don’t touch or move the Base phone.
Keep the Rover as close as possible and at least within 50 km of the Base.
Use the same model of phone for both units for best results.
When marking a corner, stand still for about 30 seconds before tapping Capture.
Use Cases
I don’t want any error correction. How do I use it?
Select “Rover Unit” mode and choose the “No Base” option from the dropdown.
Go to the point you want to capture, wait 15–30 seconds, then press “Capture”.
The app will internally record GPS every second for 30 seconds and provide TRIMMEAN , which is more accurate than a single GPS reading.
I just want accurate area and length measurements.
Choose “Setup as Base Unit” and click “Auto-fill GPS values” to auto-fill base coordinates (no need for 24-hr logging).
Start the base mode to begin broadcasting correction data.
Then, in “Rover Unit” mode, select the corresponding Base ID, go to the field, and begin capturing points. This provides accurate area and distance measurement with ease!!! Why? Because any minor error in the base values may result in shift of the KMLs but will not alter the length or area values.
I want the exact location for my points (high accuracy).
For high-precision needs (e.g., plot demarcation or permanent marking), the base station must have highly accurate coordinates. To obtain this:
Go to “Get accurate location of the Base” mode.
Leave the phone stationary under the open sky.
Let it log continuously for 24 hours.
At the end, take the averaged GPS as your reference base location. Then proceed as above, using this accurate base reference.
This method can also be used to time the average of a point of any time duration, say 10 min and get a more accurate location.
Why Smartphones Make Great DGPS Tools?
Latest phones rival dedicated GPS hardware: They combine modern Dual band GNSS chips with Assisted-GPS (A-GPS).
A-GPS benefits: A handheld GPS device needs around 60-90 sec to find out which satellites to look for getting the position. The mobile phone gets this information from the cell towers within 3-5 secs and hence helps in instant lock-on. While this may look like a minor issue, it becomes a major source of inaccuracy if the user is not a trained person and does not understand the necessity to wait for 1-2 minutes before taking GPS readings.
FAQs
Is the internet required? : Yes, to send error data from Base to Rover.
Can I use one phone for both roles? :No, both Base and Rover need to work at the same time—use two devices.
Can I use a laptop as Base? :Not usually. Laptops rarely have true GPS; using IP-based location won’t work correctly for error computation.
Do I need to install the DGPS app? :No installation required! The app is a Progressive Web App and runs in your browser on modern smartphones.
But are mobile phone GPS (GNSS) accurate enough for survey application?
Yes they are equally accurate as any other handheld GPS.
Earlier when mobile phones were using single band for GNSS then there was some difference in accuracy as the handheld GPS were using “Dual-band-multiple-satellite-GNSS”. But now when even phones have started using the same system there is essentially no difference in their accuracy at all. The specifications of GNSS for mobile phones and handheld GPS devices as given by OEMs themselves clearly show that mobile GPS using dual band GNSS capabilities have intrinsically the same accuracy compared to handheld GPS units when operating without DGPS or RTK correction.
List of Dual-band-GNSS phones that can give best results:
While this application will work and give great accuracy even on single band GNSS mobiles but for survey grade results ( error < 10 cm) dual band GNSS capable mobile phones are a must. Fortunately there are many of them across different price ranges. Here is a non-exhaustive list of some of the major dual-band GNSS smartphones including both major Android brands and Apple iPhones. This list contains all key brands—Apple, Samsung, Xiaomi, Google, OnePlus, Huawei, OPPO, Vivo, Honor, and others—reflecting devices supporting at least L1+L5 (or equivalent) GNSS for enhanced accuracy. For android phones, the direct method to check whether a particular mobile supports Dual band GNSS or not is through the GPSTest app.
The application has been tested for accuracy on OnePlus 8 using GPS (L1+L5 Dual Band), GLONASS, Galileo (E1+E5a Dual Band), Beidou, A-GPS, and the error is less than 10 cm.
For best accuracy ( ~10 cm) the same model for base and rover are advised, but otherwise the same brand phone using the same GNSS Bands and Supported Constellations should give similar accuracy.