OTISS uses Android phones, tablets and GPS devices to run the on-site survey apps for data collection. An important aspect of the on-site survey is to accurately plot the tree positions. For this you should expect to use; an accurate site/topo plan or OS map, a tape measure and also GPS.

  • Good quality phones and tablets have reasonably good GPS chips built-in and are suitable for many tree survey projects (see below). The cheaper tablets may not be as good.
  • Rugged Androids with very accurate GPS (e.g. <1metre) are more expensive than the normal phones and tablets – but they are ideal for use in outdoor environments and have very accurate GPS/GNSS.
  • See our guide to Which Android?

Even though phones/tablets don’t support high accuracy GPS – OTISS provides two methods to help.

  • First, you can read the grid references from your own GPS and enter them manually into the survey forms.
  • Second you can ‘link’ modern, accurate, Bluetooth GPS units to your phone/tablet so that the phone ignores its in-built GPS and uses the geo-spatial data supplied from the more accurate device.

Both these options are discussed below.

GPS and 3G/4G internet connection

Most modern phones have a 3G/4G chipset, a Wifi chipset and also a GPS chipset. Each of these features work totally independent of the others. Our phones cleverly use all three in combination to provide connectivity and location – and its sometimes hard to know which bit is which! The location feature in our OTISS survey apps requests a “High Accuracy” position – this instructs the phone/tablet to use GPS to locate your position on the map.

NB: GPS and 3G/4G are completely independent – so if you are in an area with a poor mobile 3G/4G signal, or you are just using a Wifi-only tablet, then the GPS/location features are still available.

Occasionally when they move to a new area of the site, the map page shows blank/pink squares because the maps were not available due to poor 4G signal. But at the same time, the red cross-hairs of the location/GPS feature is working fine. You can continue to plot the trees based solely on the GPS – with no visible map. When the 3G/4G signal improves, the maps will re-appear and the location of the plotted trees can be checked.

The above situation is not ideal but may happen from time to time. Please read the sections below and also our Downloading Maps feature to see how we work to improve these situations.

What is GPS?

GPS is the Global Positioning System, a satellite-based navigation system. There are GPS satellites run by American, European, Russian and many other governments. Most modern phones and tablets can integrate them all to get a more accurate position. Mobile devices typically use AGPS. Assisted GPS (abbreviated generally as A-GPS) is a system that often significantly improves startup performance of a GPS satellite-based positioning system. Please read the above highlighted links for more information.

The more satellites a device can monitor, the greater the accuracy it will achieve.

Android app ‘GPS Test’ by
Chartcross Ltd.

Always remember that for mobile GPS units, the accuracy depends on several factors that are independent of the quality of your device. For example, tall buildings, dense tree canopy and (to a lesser extent) the weather can all reduce the accuracy at a particular spot on a particular day. So returning to the same spot later the same day could produce a slightly different position with a better/worse accuracy value.

An accurate site/topo plan and a tape measure are will always be more accurate but a lot more time consuming than GPS. But in woodlands and away for buildings, you may need to rely on GPS for accurate surveys.

Location iconUsing GPS on your phone/tablet

When using the Map page on the survey app, press the location icon to turn the track my location feature on/off. When location is turned on, the GPS and mobile phone networks are used to centre the map at the current location (marked with a red cross). As you walk around the site, the map will automatically follow you, making it easy to plot and inspect the nearby trees and items.

Good quality phones and tablets include good quality GPS recievers that typically monitor 8-12 satellites and ‘can’ be accurate to within 3 – 5 metres (but each manufacturer and model is different).

  • There are several free GPS Test apps available that you can use to see how accurate your device is. These apps can tell you how many satellites are being used and provide an accuracy estimation.
  • Standing away from tall buildings or outside the tree canopy may get a better result. This may improve line-of-sight to more satellites.
  • Standing longer in the same position will allow the device to get a better GPS position. The position may appears to jump around a bit, but will gradually settle.
  • Note: constantly using the GPS requires more power and will use up your battery quicker.

Using your existing GPS devicegps-edit

Many of our users already have a handheld GPS device that they are familiar with. In many cases, this device may provide more accurate GPS positions than the phone/tablet.

The OTISS survey apps allow you to edit or ‘correct’ a tree’s grid reference. Use the phone’s GPS to do the initial plotting and then check the results against the handheld GPS. You may find that on some days or locations there is little difference between the two positions, but on others days you need to correct the tree positions. A better option is to use a Bluetooth enabled GPS device that automatically links to your phone/tablet (see below).

Using Bluetooth GPS/GNSS with sub metre accuracy

Special purpose GPS/GNSS devices are designed for much greater precision – from 2.5 metres to 5 centimetre accuracy. At least 2.5 metres or better is considered most good practice for tree surveying.

It is now very easy to link a very accurate external Bluetooth GPS device to your phone/tablet and benefit from the improved accuracy. Once the external GPS is configured to your phone, the OTISS survey apps (and all the other apps on your phone) simply use the ‘better’ position data provided from the external GPS rather than from the internal GPS. The OTISS survey apps show a red cross indicates the reported GPS position, and a red circle gives a rough idea of the ‘accuracy’.

To purchase one of these devices, type “Bluetooth GPS receiver” into a search engine – they are available from lots of good quality manufacturers – but always check that they are compatible with Android phones/tablets. The positional accuracy is reflected in the cost – expect to pay £100-£200 for 2.5/3m (e.g. Garmin GLO, Dual XGPS), but £600-£2000(and more!) for 1 metre and sub-metre accuracy (e.g. SX Blue, Trimble Catalyst or R1, EOS Arrow, Bad Elf GNSS Surveyor).

It is so simple!

  1. Follow the manufacturer’s instructions to link the external GPS to your phone/tablet. There is usually a new app to install on the phone/tablet (many manufacturers recommend the free “Bluetooth GPS” app from GG ModLabs) and you may have to enable an Android feature called “MockGPS”. Make sure to follow the manufacturer’s instructions.
  2. When surveying,
    • Turn on the GPS device and put it in your pocket,
    • Use the Bluetooth GPS app to connect to your GPS device – it will display your current location.
    • Leave the phone/tablet’s Location feature enabled. While connected to your external GPS, the Location feature will use the ‘mock GPS’ position provided via Bluetooth, rather than powering up the internal GPS.
    • The OTISS survey apps will automatically use the more accurate GPS position from the external GPS. There is nothing to change/configure on the OTISS app. This is just another feature of your phone that makes surveying easier with OTISS survey apps.
  3. There is an added benefit that your phone/tablet uses less battery power because it does not need to use its internal GPS. A Bluetooth link uses very little power compared to the GPS. You will see that the usual GPS icon does not appear on the phone’s status bar, instead the Bluetooth GPS app icon is shown.