Global Positioning System

The Countryside Service uses GPS (Global Positioning System) to map habitats and other features for management. The data is stored and manipulated by the County Council's GIS (Geographic Information System).

GPS greatly speeds up the mapping of species and habitat information... current projects include monitoring of scrub and tor grass Brachypodium pinnatum invasion of chalk downland at Pitt Down (Winchester), and National Vegetation Classification at Lymington/Keyhaven marshes.

GPS has also been used recently to map Danebury Iron Age Hill Fort, West Down Chilbolton, Yateley Common Country Park, and Crab Wood. Colleagues in Environment have also made use of the system, and English Heritage rediscovered an archaeological site in the New Forest amongst relatively featureless heath by using our GPS. A habitat map of West Down was produced in just 3 hours surveying.

GPS also lends itself to Geocaching, a new countryside based pursuit, which uses domestic navigational GPS units to explore the countryside.

Global Positioning System - GPS and the Countryside Manager

The following article was originally reproduced in the "Ranger", the journal of the Countryside Management Association, in June 2002.

GPS (Global Positioning System) is now commonplace parlance amongst land managers. Phil Allen explains how Hampshire County Council's Countryside Service has used GPS to map and manage its countryside estate.

How does it work?
GPS is an American military system, consisting of 24 satellites which orbit the earth in very precise paths. A receiver on the ground picking up signals from a minimum of four satellites will be placed accurately on a mathematically perfect ellipsoidal model of the earth, the so called WGS84. Because the earth is not a perfect ellipsoid, most developed countries have worked out the local variations, and modern GPS units will convert the WGS84 data into the local projection, eg OSGB36 which gives a UK National Grid reference.

The way it works is actually quite simple. The receiver is updated with information of all the satellites' positions at any time in the orbit, and its internal clock is set to match the atomic clocks in the satellites. A signal is broadcast that matches one generated internally by the receiver, and the receiver is able to calculate the time difference between the generation and reception. Since the speed of the signal is known, it is an easy matter to calculate the distance to the satellite. Given 4 satellites of known position relative to the globe, the receiver's horizontal and vertical position on the WGS84 model is calculated by the receiver's software. If the user requires it, and the model is available and installed, the data is transformed to the local projection; so in the UK a grid reference and height above mean sea level (Newlyn) is displayed.

How accurate is it?
Accuracy depends very much on the build quality and software sophistication of the receiver. At the bottom end of the market you can buy navigational hand held units that will perform very adequately in open country. You can even programme some of these units with navigational nodes (waypoints) to guide you along a pre-planned route. 20m accuracy is fine for this kind of application. At the top end, civil engineers use GPS units that are accurate to 1cm, but we are talking serious money here! The units we use at HCC are 1-5m accurate in normal use, but can be used with software and other correction techniques to get down to 10cm. We employ this kind of accuracy for fixed point photography, setting the unit over a small piece of scaffold tube driven below the ground. Quick relocation is easy, initially with lower accuracy GPS, and then a small metal detector. Using Trimble's GeoExplorer handheld GPS, Tony Davis has mapped heather age classes at Yateley Common Country Park in north Hampshire. Once the heather areas were put together in a GIS, he found that the tracks and paths that intersect the common did not need to be separately surveyed, because the edges of each discrete heather area linked together so smoothly to show the edge of the tracks. That's perfectly good enough accuracy for most of our land management needs.

A common misunderstanding is that a navigational handheld will produce a fully featured map for you…..well, it will if you note down the waypoints and plot them in the office, or plug it into a field computer or palmtop that will run real time GIS. For habitat mapping a datalogger is much better option. This device is self contained, and will collect point, line, and area features, complete with the attributes and attribute values, and then download the whole into a GIS. Some loggers will intelligently add to, or update an existing GIS file.

Error correction software is important. Signals can get bounced off other features before reaching the receiver, known as multipath. MaxTrax (Leica) and Everest (Trimble) are examples of proprietary software that filter multipath errors, and facilitate using GPS in difficult conditions. You can also use differential correction, in real time or back in the office. The principle of differential correction is that a base station of known position logs the same data as the roving receiver. The base applies an error correction vector if it receives signals that place it outside its known position (usually atmospheric noise or satellite orbit aberrations). These corrections are then applied to the corresponding rover points (by time matching) to achieve greater accuracy. In Hampshire we are lucky that the Solent coastguard broadcasts correction data. If you are outside the range of one of these stations, you can set a rover up as a base unit on a known point.

Having run a course for the local Agricultural College on the use of GPS, I would say that the biggest error factor in mapping is operator insensitivity. Once students were able to see the effect of not focussing on keeping the unit stable, the results improved dramatically. The power of the satellite transmission is the equivalent of a candle on the moon…..don't make the receiver's job more difficult by swinging it round like it's a personal stereo!

Why not use aerial photos and conventional maps?
Aerial photographs are an efficient data capture tool at large scale, but may not have the required resolution or be flown frequently enough to map habitats that cycle relatively quickly, like heath or coppice. The OS do not map rural areas to the extent of showing small habitat management units, and there are copyright issues. Given steady access to GPS, a manager of natural areas can monitor areas as frequently as required by a management plan, focussing on their skill as land managers rather than the process of surveying.

What are the practical issues?
Fig 1 shows the raw output from the GPS survey of a 0.2 ha coppice area, showing major trees and canopy spread, coppice density sweeps, paths and woodbanks, and all young retained saplings and pole stage maidens. The red circles are 95% confidence limits of the GPS point, and the tick marks are 25m intervals. Software filters the worst data out to the users specification, and the whole survey is complete in 3 hours.

The map in Fig 2 is of a 20ha downland and woodland SAM. Mapping took two 4 hour site visits, and 1 day error filtering and GIS editing. We now have a record of paths, car parks, fences, dangerous trees, and 8 habitat types. With good satellite coverage and ideal conditions, data is often clean enough to export directly to GIS. Using GIS as a database tool, records can be queried and grouped for actions within a management plan.

Some important observations over the years

• Plan your survey to coincide with maximum satellite coverage
• Plan very carefully what you want to record before you go out in the field to collect data
• Plan a file system for your data, and make sure every user sticks to it
• Archive the raw GPS data, and back up your main GIS files frequently
• Big wet beech canopies in summer are a GPS blackhole….other species are OK

Future Developments
The Ordnance Survey have worked hard at improving the accuracy of their mapping, and have recognised the increased use of GPS. To this end they have new GPS standards, and transformation models (OSTN97 and OSGM91). Their aim is get everyone using these models for data sharing and compatibility. The Russians (GLONASS) and Chinese (BNTS) have GPS-style satellites in orbit, and soon the European Space Agency will launch the first of its navigation satellites, the Galileo project. Galileo will provide high accuracy 2 channel GPS, with options to receive transmissions from other GPS systems. Its aim is a civil one, in particular to assist with such things as emergency services and aircraft movements. Accuracy and reliability standards are said to be much better than anything currently available.

What's on the Market?
GPS World lists 61 GPS manufacturers. "Household" names include Garmin, Magalen, Furuno, Motorola, Matsushita, Leica, Sokkia, and Trimble. The latter 3 certainly manufacture high spec. data loggers that are aimed at the land management sector. Price is always an issue, and of course you get what you pay for. You should look for error reduction software, OS model transformations, data import and export flexibility, and multi-satellite tracking, as well as functionality in the field.