Note 8: Plant records, GPS usage, datum and Google Maps by Mark Hyde

This note sets out in detail the system used for recording plant positions and introduces a change to the system from today (3 August 2008).

The plant recording done in Zimbabwe up to today (3 August 2008) has been done as follows. Before the purchase of a GPS (c.2000?), records were made using the UTM system and read off the 1:50,000 series maps produced by Zimbabwe's Surveyor-General.

These were mostly done to 6-figure accuracy. For example, UR330182. This is actually a small square with a 100 m side, so the level of accuracy is not perfect.

With the advent of a GPS (a Garmin GPS48), recording continued using the 6-figure UTM system. The datum used was ARC 1950 which gives good agreement with the 1:50,000 maps of Zimbabwe.

In order to plot the UTM points using Google Maps, a program called phpcoord (from the internet) was downloaded and used to convert the UTM grid references to latitude and longitude values.

Agreement with the actual position of the plant again is only approximate because of the somewhat crude nature of the 6-figure position.

During a recent visit to the UK, the plant records were made using the same GPS machine but using the WGS84 datum and recording in latitude and longitudes to 5 decimal places.

The results were excellent with often very good agreement between the actual position and that shown on Google Maps.

Today, Mark tested 4 datum systems (ARC 1950, ARC 1960, Cape and WGS84) in his garden here in Harare against the latitude and longitude as on Google Maps. The WGS84 datum again gave excellent agreement and this system will be used for all recording in future, where possible.