02 December, 2009

Tools and Tricks of the Trade - the GPS

Ahead of fieldwork for my thesis I asked what kind of GPS device is recommmendable. A number of fellow geobloggers replied with valuable advise. I actually went and bought a GPS ahead of fieldwork. Now I am a proud owner of a GPSmap 60CSx.  The device has fullfilled all expectations in terms of durability, endurance and accuracy under not perfect conditions. The batteries last for 3 days of work or more. Dropping it on rock doesn't cause a scratch and signal reception under dense tree cover is excellent. As a matter of fact I still had a signal when entering underground caves. Well, at least for the first two or three meters. But I don't want to make a commercial here. Fellow geobloggers will know, non-trained readers might wonder though what I do with a GPS device. The idea is pretty simple. It allows to rapidly map and document geological relationships in the field with sufficient accuracy. Mapping lithological borders i.e. becomes a piece of cake. Find the contact in 3 or 4 locations, take a measurement of Easting and Northing, if possible also take a dip measurement with your geologists compass, and you are done. At least in principle. You don't necessarily need the expensive GPS device I decided to buy. A price of 260 Euros is not cheap. There are cheaper alternatives or even more expensive ones with a great variety of additional gadgets.

The GPSmap 60CSx operating inside

Using a GPS is easy. If you do not get training at University the documentation should allow you to catch on easily. The basic steps are simple. When the accurate position is your main or only interest, as it may be for most geological fieldwork, there is not much to do. After activating the device give it a moment to acquire a satellite signal. Depending on the location and ground conditions this should not take longer than 1 minute with a good device. Next select the position format and map datum. Usually these are hidden in the units or settings options of your device. In the most simpliest sense the position format determines the way how you write down your coordinates. As the Earth is an irregular globe your need a projection to realistically depict the surface in a flat, 2D map. One of the most common formats is the UTM (Universal Transverse Mercator) format that devides the surface into boxes. The map datum is the reference system. Because our planet is not a perfect globe you need a different model of the surface depending on your position. A model that best fits the real shape of the planet. A commonly used datum is the WGS84. Depending on your location you may wish to use another format and datum. In Germany you will still frequently encounter the Gauss-Krüger-coordinate system combined with the Potsdam-Datum or the Bessel-Ellipsoid (Bessel 1841). It fits the Geoid in Europe exceptionally well.

Having decided on your position format and map datum you can start mapping. Either simply noting the coordinates into your fieldbook or saving way points for download in the evening. When using way points remember to always save the way point and not to forget one. When working on large areas way points will make it easier to track back your movements.


hypocentre said...

I've got an 60CSx too - it is excellent (I did manage to break the catch on the battery cover by dropping it). I'm now considering an Oregon 550 as you can load your own basemaps into it.

More to the point, now you have a 60CSx, aren't you going to have to change the name of the blog?


I have been using a Garmin etrex for about 9 years now. It's never very good in the woods & sometimes even out in the open, I get readings that are compared to Google Earth way off.

Maybe it's time to get a newer model.

Lost Geologist said...

You have a point hypocentre! Maybe I will think about it.

Aydin, with the current device I can get reading in the open of an accurary down to 3 m. In the woods with tree cover 5 to 10 m are normal.

Miguel Vera said...

Seems like the 60CSx is very popular, I know several geologists that have one. I think it's the best you can buy for the "lowest" price possible.

I've had an etrex for 3 years now, and I'm still happy with it.

Anonymous said...

I'm beginning an M.Sc program here in Nigeria in March. Going to the feild last week with a friend I experienced the signal problems using Garmin etrex legend.

I want to get my own GPS and 60CSx (from your nice commercial) seems to be it. 260 Euros is a lot of money though. For how much did you get the compass and UV lamp?

Lost Geologist said...

The UV lamp is around 30 to 35 Euros. The compasss on the contrary is hilariously expensive and I only bought it because I recently got a big check from the company sponsoring my master thesis. I recommend only getting one when you really feel you have money to spend. The Freiberg Compass costs almost twice as much as the GPS did. It is a very reliable precision instrument though and virtually indestructible. With borrowed compasses I used to break rocks when not having my hammer in hand's reach. It had a reason why I waited to buy one until my study time is almost over.

The GPS has an integrated compass in it. In very well exposed layers you can at least get a strike measurement with it as it will tell you when it is being held horizontal.

Anonymous said...

Double the price?! (520 euros or more than 100000Naira!). I think I may just go for the 50euros plastic type i see a lot around here...

Thnaks for the suggestions

Lost Geologist said...

You are totally right. The price is...ehemmm...insane. I paid 498 Euros to be exact. And, as said, I'd only get one when you get your first, big pay-check. As a matter of fact I managed all my student life without one and borrowed it when needed. So there is no hurry.