Remote Digital Mapping: A User Guide to Find Rock Orientation
By Les Hasbargen, Associate Professor in Earth and Atmospheric Sciences Dept., SUNY College at Oneonta, New York
This page outlines a method for collecting strikes and dips from aerial images and elevation data. At the core of digital mapping are 3-dimensional coordinates, and once one has these, the tools of vector algebra can be used to find strike and dip of geologic surfaces. An analytic solution to the 3-point method underlies operations in the Excel worksheet below (follow link). The basic method is: collect coordinates of at least three points on a surface of interest, transfer the points to the spreadsheet, compute the strike and dip, and output the data to a file readable by Global Mapper, a spatial mapping software. One can alter the macros in Excel for writing the data to suit their own needs. If you are interested in the paper which describes in greater detail the three point vector method, contact me (Leslie.Hasbargen@oneonta.edu ) and I will provide a link to the paper publisher (Geological Society of America). They give me 50 free reprints in pdf format to distribute to those who cannot afford to purchase the article.
The video tutorials below are flash video (.swf), and clicking on them pulls up a web page which runs the video. The videos are currently voiceless and rough. They record in real time the steps to get the data and process it. Be patient with the pace, and pay attention to the cursor. Also note that in some places, the mouse has been “right-clicked” to pull up a list of available operations. The software used in the process of mapping geologic features includes GIS (Google Earth and Global Mapper), a macro-enabled spreadsheet (Microsoft Excel), and a web browser (Google Chrome).
1) Obtain elevation and aerial imagery for an area of interest.
a)
From USGS National
Map Data Server: http://nationalmap.gov/viewer.html
i)
Download and save
the USGS elevation data (NED, 1 arc second and 1/3 arc second spacing), and the
ii)
National
Agricultural Aerial Imagery Program data (NAIP, at 1 m spacing)
iii)
Click here for a video tutorial.
Note that the former data server for the USGS, the Seamless Server, has been
deactivated as of December 2012. The data have migrated to the National Map,
and the USGS, for whatever crazy reason, has opted for tiled access to the
data. Gone are the days of selecting an area of interest, and extracting only
that data. Sigh. All good things must come to end. Anyway, the data are still available.
You will want to zoom to an area fully to minimize the tile size. My first
attempts led to downloads of over 300 mb. Annoying,
and wasteful, but the data are there…
b)
From Global
Mapper (or ArcGIS) using Web Mapping Services (WMS) providers (free!)
i)
Use the Download
Online Data tool
ii)
Select the data
and set the boundaries of the region of interest to download (several options
here…)
iii)
You will save
yourself a lot of time if, once you get the data into Global Mapper from the
WMS, you export the data to a local drive. Global Mapper reloads the data from
the WMS every time a change is made, and the reload will take time. Note that
if you try to download a large area at high density, you’ll freeze the program.
So, moderation and patience are your best allies!
iv)
Click here for a video tutorial in
Global Mapper.
2) Convert the projection of the data to an orthogonal reference frame, such as Universal Transverse Mercator. Click here for a video tutorial in Global Mapper. If you collect coordinates in Google Earth, go to Tools/Options/ShowLatLon and select Universal Transverse Mercator.
3) Collect coordinates for at least three points on geologic surfaces in a GIS software.
a)
Within Google
Earth:
i)
Use the path
profile tool to create lines with at least 3 points. Google Earth provides
perspective views on the fly, and you can sketch contacts, erase unwanted
points (use Delete), and move points, even after the line is saved. To
reactivate the line, right click the name of the line in the folder, and select
Properties. The line will be activated and editable. Save each line in a
folder, and then save the folder as a kmz file. Click
here for a video
tutorial.
ii)
Open the kmz point file into a separate GIS, where the horizontal
coordinates can be combined with elevation, and exported as a text file. Click here for a video
tutorial.
b)
Within Global
Mapper: Click here for a video tutorial.
Note this file is a bit large, but provides a good view of the process of
mapping three points on a contact (bedding surface, in this case).
4) Read the three-point lines into a spreadsheet and determine strike and dip. Click here for a video tutorial.
5) Export the strike and dip data from the spreadsheet back to the spatial data software. Click here for video tutorial.
Hasbargen, Leslie E., 2012, A test of the three point vector method to determine strike and dip utilizing digital aerial imagery and topography, Chapter 14 in Geological Society of America Special Paper 492, Google Earth and Virtual Visualizations in Geoscience Education and Research, edited by Steven J. Whitmeyer, John E. Bailey, Declan G. De Paor, and Tina Ornduff, SPE492: ISBN 978-0-8137-2492-8.
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Tutorials created by Les Hasbargen, Earth &
Atmospheric Sciences, SUNY College at Oneonta
Last updated: November 18, 2013
Created: December 4, 2011
Email: Leslie.Hasbargen@oneonta.edu