The very first thing that we need to do is to enable the GPS device in the Settings screen. Note, that when we create a new project the GPS device is turned on by default. Then, we go to the “Points” screen and we click on the “+” button (in the top right corner) to add a point.
How does a GPS device work?
All smartphone (and few tablet) devices are equipped with a GPS device (or GPS receiver). This device communicates with satellites to determine our location on the globe. We do not want to get into details about how a GPS device determines the location, but, what matters is that it has to communicate with at least 4 satellites. This means that it takes some time to communicate with them, and thus we should be patient when we see a message saying that the GPS device is not ready!
The accuracy of our location is determined on the quality of the signal that the GPS device receives from the satellites. To alleviate the signal problem, the smartphones determine our location by cell tower triangulation and WiFi hotspot locations. Thus, if we have a 4G signal on our smartphone, it is almost certain that our location is determined with a good accuracy.
Add a point
We add points in the “Points” screen, by clicking on the “+” button. Now, in some cases we may see a message saying that the GPS device is not ready yet. This usually occurs after we add a point, and it means that iTopography is trying to get a new location from the GPS device. After a few seconds, we should be able to add a new point. If not, then it would help to move to a new position or to open the map application.
Each point in the list has the following information:
- Altitude: How far from the sea level we are located. It is in the Units that we set in the Settings screen
- Accuracy: Accuracy of the GPS device, in the Units that we set in the Settings screen
The accuracy of each point will be less or equal to the accuracy that we set in the Settings screen.
How is the Accuracy in the Settings screen used for?
The accuracy in the Settings screen
When we enable the GPS device in the Settings screen, we immediately see a new entry, prompting us to select the accuracy. This is not the accuracy of the GPS device, but the accuracy of our calculations.
We set the accuracy in the Settings screen for two reasons:
- Control the data that we get from the GPS device
- Control the accuracy of the calculations
Control the data that we get from the GPS device
We have no control on the accuracy of the GPS device. iTopography is constantly acquiring location from the GPS device and is reporting its accuracy in the “Points” screen. In the next screenshots, the accuracy is set to <10 m (less than 10 meters).
The background of the GPS status depends on the accuracy of the GPS device:
- Red background: The accuracy of the GPS device is more than 4 times the selected accuracy in the Settings screen
- Orange background: The accuracy of the GPS device is less than 4 times but more than the selected accuracy in the Settings screen
- No GPS status: If the accuracy of the GPS device is less or equal to the selected accuracy in the Settings screen, the GPS status disappears, and we now have the buttons to modify the points
Once the GPS status disappears, we can click on the “+” button to add a point.
Note, that if we set the accuracy in the Settings screen to 100m, the accuracy of the GPS device can still be better, i.e., 5m. Most often we would set the accuracy to <10m, but, in some rare cases, the accuracy of the GPS device will not be that good. When this happens, we need to set it to a larger value in the Settings screen.
Control the accuracy of the calculations
We should consider the accuracy as a circle that encloses our actual location. The value of the accuracy denotes the radius of that circle. We do not know where we are in the circle, all that we know, is that we are somewhere in that circle.
Hence, when the GPS device determines our location with accuracy of 100m, this means that we are located somewhere in a circle with radius of 100m. That circle has a surface of 3.14 * 10km2, and that is a very big surface!! We shouldn’t trust the calculations with such large accuracy.
In most times, the accuracy of the GPS device is 3 – 5 m, which means that the surface of that circle is 3.14 * 25m2. If we want to compute the surface of a square field whose side is about 100m, we would have computed it with a good accuracy.
We always set the accuracy in the Settings screen to <10m. If the GPS device can not determine our location with that accuracy, we increase it to 10m, and we keep doing that until we get our location.
When we add a point or we move to a new position, we need to wait for a few seconds until we add a new point. This happens because we wait from the GPS device to acquire a new location from the satellites. It should not take more than a few seconds.
When we add a point, before we move to the new position it is wise to add a new point. If the GPS device is able to determine our location with a better accuracy, we can delete the previous point. If not, we will see message informing us that we have already added this point.