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Geolocation Accuracy Tips for GLOBE Observer tools: Mosquito Habitat Mapper and Land Cover


In this pro tip, you will learn how to increase your location accuracy while using the GLOBE Observer app and learn why location accuracy matters.

How is location determined?

By using a Global Positioning System (GPS) receiver, (either  hand-held or built into your mobile device), you are able to obtain your geographical position on the Earth’s surface. The antennas on your GPS receiver will acquire satellite signals and once it has located three satellites, it will perform a first calculation of latitude and longitude.  For the GLOBE Observer Land Cover tool,  the initial estimate of your location may have more uncertainty (+/-65m). As more satellites are located and positions are more precisely known in space, your location accuracy increases.

Why does an accurate location measurement matter?

Satellites provide global coverage and enable quantification of changes in the Earth system on a global scale.  For some questions, however, scientists need more detail than a satellite can see. GLOBE Observer is helping fill this need for detailed observations on the ground. Citizen scientists are using the GLOBE Observer app to make observations of land use, land cover, and the standing water microenvironments where mosquitoes lay their eggs. It is important to obtain an accurate location measurement so that data collected in different ways – in this case by satellites and citizen scientists – can be matched and compared. 

1. Obtain the most accurate GPS location using the GPS in your mobile device

Screenshot: Land Cover tool

You will see a bar under the map, “estimated accuracy.” When you open the app, you will immediately see an estimated accuracy, such as 65 meters.  Wait a few seconds? and then touch the “reset” icon (on the iPhone-  some Androids reset accuracy automatically). Do this at least once. Ideally, the accuracy will improve significantly, to ca. 5-10 meters or less.

Why: The GPS receiver in your phone may require several minutes to acquire data from a sufficient number of satellites. The time it takes to secure the signal will vary, depending on the availability of GPS satellites, cellular towers, obstructions such as canopy cover, land forms or buildings, and battery strength.  If obstructions prevent you from obtaining the GPS accuracy you want, you can try to move your location.

Pro tip: If you are doing a Mosquito Habitat Mapper measurement and need to report the accuracy of your location, you can open GLOBE Observer Land Cover, obtain the latitude and longitude and accuracy. Write down this information. There is a comments box on the last screen of the Mosquito Habitat Mapper (see image below) where you can identify the positional error estimate for your own use.

2. Using the map to improve your location.

It is almost always better to report the location provided by the GPS in your mobile device.  However, if you keep hitting the reset button next to the estimated accuracy bar in the GLOBE Observer Land Cover and are not achieving an estimated accuracy greater than 30 meters, using the manual map location may be the best choice. Using two fingers, move the map so that the point where you are standing is correct on the map.

3. Using the GLOBE Observer app in offline mode

Question: I want to make an observation, but I have low connectivity and the app is trying to automatically update.

If the app is trying to automatically update when there  is low connectivity, it may prevent you from launching the app. A work around for this situation is to turn your phone to airplane mode, then open the app. This will prevent the app from trying to update, so you can make your observations. The app works off line and you can upload your observations when you are back online.

Screenshot: Mosquito Habitat Mapper

You can also use the GO app in  airplane mode to save your phone battery. However, be aware that If you are in airplane mode, you are getting location information only from satellites, and cell towers, which normally serve as additional nodes to help narrow down the location, and these will not be available to you.

4. Pro tip: Obtain the most accurate GPS location using a dedicated GPS receiver

Note that when  you are using the GLOBE Observer app in offline mode, there is no map served up in the app to confirm your location coordinates. This is one circumstance when you may wish  to employ a dedicated GPS unit or a navigation app that has off-line map capability.

Handheld devices will “ping” satellites more often, and typically will return a location with greater accuracy, usually to 5 m. this is because there are more antennas in a dedicated GPS device. Many handheld GPS receivers use Bluetooth technology to  pair with the GLOBE Observer app, importing the GPS coordinates directly into the GLOBE Observer app interface (consult your device manual for information).

5. How to access accuracy information in GLOBE Observer data

It may be important to the analysis of your data to know the range of error of the measurements. This data is currently not reported on  data downloads using the GLOBE Advanced Data Access Tool (ADAT), but can be accessed through data acquired through the GLOBE API for the GO Land Cover app.

Last,  most GPS receivers in cell phones have an accuracy of +/- 5m,  depending on whether there are physical barriers or weather conditions that interfere with the GPS signal. Normally, precision of the location signal is indicated by the number of digits after the decimal point. In the GLOBE Observer app, the number of digits after the decimal point in the measurement is set by default to 4 places (.0000), even if the measurement is not that precise. You will need to obtain the range or error from the GLOBE API to confirm the precision of your observation.

To learn more about the use of a GPS receiver, consult the GPS Protocol in the GLOBE Teachers' Guide.

Co-author Peder Nelson, Oregon State University, is the science lead for GLOBE Observer Land Cover.

 

 

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