Wednesday, March 25, 2009

Wedge #16: A Bit on GPS Mapping

This post is a submission to the March Accretionary Wedge, being hosted by Kenneth Clark at the Office of Redundancy Office, which is on recent geologic, geological, or scientific advancements.

Over the years there have been numerous advancements that have affected my interests and the way I do my work. Some of these have related directly to my geological interests (discovery and recognition of Carlin-type deposits, prior to my move to Nevada, would be one example). Others have related more to the way we understand geology in a broad sense, for example plate tectonics, which is specifically off-limits for this Wedge. The changes in computer technology over my entire career so far would be an example of advancements that have affected the way I do my work and the way we all do geology in general - from dropping cards in card-readers full of Fortran and sending the program to the mainframe, to our first moves into personal computers with early IBM's, Compaqs, and Macs, to the present use of various high-speed (until tomorrow) laptops and mobile computing devices.

Recently, the use of GPS devices has directly affected the way I do mapping. Prior to 2005 or 2006, I mapped solely on paper with no electronic support. I preferred enlarged air-photo base maps, and transferred my geology to topography using a triangle method that was involved and time-consuming. My recent forays into using a GPS while mapping have rapidly evolved to where a GPS unit is required.

GPS use while mapping and sampling can increase the amount of work one person can do tremendously, and it can also increase the detail of the work, depending on scale and time. When sampling, it's fairly easy to take a reading, take a sample, rewrite the GPS waypoint label with the sample number, and move on to the next sample. The location is more precise than the former dot on the map, and maps can be plotted directly from raw or fairly raw GPS data at any scale. It's probably best practice to still plot the sample location on a map while in the field, as a kind of backup, and a tertiary backup would be to write down the GPS northing and easting in your field book. For fast work, though, the second step is sometimes done in the afternoon or evening, and the third step is often skipped entirely.

I still write down many GPS waypoint labels or numbers in my field book, with attached notes that include the azimuth and angle information of structures, bedding, and other orientations, along with any notes such as locations of digital photos. I rarely write down the GPS readings - the northings and eastings - but for backup purposes, as mentioned above, these data would be useful if the GPS totally failed en route to the motel or office.

I recently discovered how easy outcrop mapping can be when using a GPS. An outcrop map is moderately common in exploration and mining. An outcrop map can be made on any type base map, including an air-photo base, a topographic base, or a grid base. With a GPS, one can walk around the area of outcrop taking points every so often, the frequency of point-taking depending on scale of the map. Using a grid sheet, one can then plot up the points and draw in the outcrop. One can then add in bedding, structure, contacts, and other information on the mapped-in outcrops, taking GPS readings for all orientation measurements. GPS waypoint numbers or labels are easily converted to actual azimuth and angle readings - either immediately or later - with an initial letter symbol defining the point number as bedding, fault, joint, cleavage, fold axis, or other. A GPS point number or label could be B07525, indicating a bedding strike of 075 or N75E, with a dip of 25 degrees to the SE. One uses a right-hand or left-hand rule for determining strike and dip, and you have to be sure everyone knows what you mean by right-hand or left-hand (and see the comments).

What I found is that this method requires GPS and grid-sheet only; no other base map besides the grid sheet is required: you don't need to have any grid-lines marked on the ground, you don't need to have any air-photos, and you don't need to have any topographic maps in hand. The outcrops as plotted from GPS data onto the grid sheet become the base map. It can be useful to have an air-photo or topographic map to look at or examine while in the field; these are not needed, except for some kinds of interpretation, and mapping goes much faster without them.

I do feel lost when getting started with this method, however, probably because I still miss topo lines or air-photo trees. I stare at the grid sheet of two dimensions while the large three-dimensional world looms all around me. It's a little like trying to write on a fresh, white sheet of paper, which gives me a form of writers block I elude by using yellow paper, or by typing as fast as possible on a computer. If you want to avoid white-paper syndrome while starting to map on a faintly blue-lined white sheet of grid paper, plot up some points as fast as possible - or, better yet, spill some coffee or dirt across your map.

UPDATE: This Wedge was not published, hence it isn't #16 as stated above.

2 comments:

Lost Geologist said...

GPS mapping is really useful. I work like that all the time. We hardly even learn triangulation of points at uni anymore. I do not safe waypoints in the GPS though. I am always making analog paper maps for my uni/work and waypoints are almost useless for me. So I note down the coordinates in the fieldbook together with important notes on lithology, stratigraphy, strike and dip. Without GPS I would be totally lost in my current thesis mapping. The topo-maps are in parts totally wrong, roads and other orientation points do not exist, and roads that exist are not on the maps. 80% of my area being woods with no reference points GPS safes the day.

Silver Fox said...

I think things are a lot easier overall with maps, personally. With the form of outcrop mapping I was doing, I got used to not having any clues besides the real world and the GPS. It was occasionally disconcerting, but mostly faster.

And the points and tracks were all downloaded directly to my computer, and from there directly into various map-making programs.