Keep Your GPS on Track

This explainer post from the AgTerra Academy provides a deep dive into GPS technology and how you can make use of the unique features of MapItFast to get the most out of your mobile device’s built-in GPS receiver.

GPS positioning involves communication between a GPS receiver on the ground and a network of satellites all around the globe. Almost all modern mobile devices have a built-in GPS receiver, which forms the foundation of MapItFast’s mapping functionality. Using just the phone or tablet you’re already carrying around, you can create accurate, detailed maps of your field activity with a single tap.

While mobile GPS positioning is a marvel of modern technology, it’s still an approximate science. When the GPS receiver in your mobile device reports a location to MapItFast, it also includes an accuracy report. The reported accuracy is a function of the number of satellites the receiver is connected to among other factors. When you map a point, line or polygon, MapItFast marks a location on the map that corresponds to the location reported by the device GPS. But your true location could be anywhere within the reported accuracy distance from that mark.

MapItFast displays GPS accuracy at the top left of the map screen on your mobile device. If you watch this display for a while, you will notice the accuracy fluctuating up and down. The number is usually very high when you first open MapItFast as the GPS starts acquiring signals from satellites. It will settle down after a few minutes but will continue to adjust as the receiver loses the connection with one satellite and picks up another.

The accuracy value tells you how close your true location is to the reported value. The lower the value, the more accurate the report.

Because GPS positioning is approximate and variable, MapItFast won’t always draw a line that follows your route precisely. It may include some zigs and zags as your reported location jumps around according to the signals the GPS receives. However, there are some things you can do to reduce the effects of GPS limitations on your mapping. We’ll outline some of these here.

Upgrade Your Mobile GPS

MapItFast can only work with the location information that it receives, and not all GPS receivers are created equal. High-quality mobile GPS receivers will often achieve an accuracy of 3-15 ft in optimal conditions, which is sufficiently accurate for most field mapping activities.

If you are consistently seeing accuracy values much higher than this, then you might consider upgrading your GPS receiver. Unfortunately, this means upgrading your mobile device.

Super-Quick GPS Explainer

A GPS receiver on the ground gets a timestamped signal from a satellite with the satellite’s known location. Using its internal clock, the receiver knows how long it took the signal to reach it.

Then, the receiver can use math and science to calculate an approximate position on the Earth’s surface.

Since the Earth is (roughly) a sphere, there are a lot of locations on the surface that are the same distance from the satellite. The receiver can’t know which point it’s at for sure.

But by getting a signal from multiple satellites, the receiver can triangulate its location highly accurately. The more signals it receives the more narrowly it can identify its location.

However, there are many affordable tablets out there with high-quality GPS receivers built in. For Android devices, AgTerra recommends the Samsung Galaxy S phones and Tab S tablets or the Lenovo Smart Tab M8. We advise against the Samsung Galaxy J and Tab A series devices, as these are known to have limited GPS capabilities.

Despite their many virtues, iPhones and iPads are not known for the quality of their GPS receivers. MapItFast works well on Apple devices, but if you need high-precision mapping, we recommend using an Android device. Also note that Apple makes a Wi-Fi-only iPad, which is not GPS capable. This model will not work with MapItFast.

Using SprayLogger GPS

Some SprayLogger customers will notice a checkbox in the lower SprayLogger panel that says Use SprayLogger GPS. The SprayLogger E3 series dataloggers have a built-in GPS receiver and come with an antenna that can be mounted on the outside of your spray rig. You have the option of using the MapItFast GPS reports or those collected directly by SprayLogger.

The GPS report displayed at the top left of the map screen comes from your device GPS even when you select the Use SprayLogger GPS checkbox. The SprayLogger GPS is used only to draw spray and no spray lines on the map during a spray job.

Give Your Device a Clear View of the Sky

Mobile devices that are stowed in a pocket, backpack, or truck cab while mapping can show reduced accuracy. For optimal GPS conditions, the device should be held high and clear from anything obstructing its view of the sky.

GPS accuracy is also negatively impacted by the environmental conditions in which you’re using the device. On overcast days or around tall buildings, trees or cliffs, satellite signals can be blocked from reaching the receiver, thus reducing the accuracy of its reading.

Unfortunately, when you need to get field work done, you may not be able to avoid these environmental conditions. If you notice that you are getting low accuracy while you are mapping, we suggest that you note the objects that were mapped with poor accuracy. You can use the description field of your mapped objects to indicate that those objects should be inspected and potentially adjusted back at the office.

Adjust the Location Reporting Engine

Adjust the GPS Recording Constraints

Oftentimes, no data is better than bad data. If poor GPS accuracy means that MapItFast will record you at a location that you aren’t actually at, it might be better to instruct MapItFast not to record the location at all.

You can tell MapItFast to ignore bad data by only recording locations when the accuracy meets a set standard. You tell MapItFast what your standard of accuracy is, and it will record only when your device achieves that standard. Details on how to set up accuracy constraints on your device.

Note that when recording lines and polygons, MapItFast will skip points when the accuracy is too low. The result is a “straight line” connecting the two most recent recordings. When you see a straight line where you actually traveled on a curve, this is due to poor accuracy reading. If you set the accuracy limit lower than your device is able to achieve, then you will never record your location, and you will be unable to map objects.

You can also tell MapItFast to record no matter what the accuracy. The result is often a jagged line that shows every instance of your GPS location jumping around.

Edit Object from the Web

It is possible to correct for occasional GPS issues after the fact by editing mapped objects using the MapItFast web app. If you know that a marked point is not at its true location, it can be repositioned. You can use the features on the basemap as a visual guide for repositioning objects.

With lines and polygons, every GPS recording that MapItFast makes while mapping the object is saved as a point along the line or polygon perimeter. Each of these points can be altered to correct for GPS blips in the object. You can move individual points or delete errant or unneeded ones. You can even add new points to a line or polygon. This can be useful to add in positions where MapItFast drew a straight line because GPS accuracy was too low.

For more details on editing map objects online, go here.

Now Get Out and Map!

Now that you know a little bit more about GPS and how MapItFast uses it to map your field work, head out and start creating detailed maps of whatever it is you do. And if you need some more help getting started, reach out to us for more tips and tricks.