Visualizing dip and strike using Google Earth

Barbara Tewksbury (Hamilton College)

Inclined layers on the flank of a plunging anticline near Khvorgu, Iran (27 33 48.03N, 56 26 24.71E)
Once students have done their first mapping exercise and have a good visual grasp of inclined units and contacts, they explore the south flank of a spectacular plunging anticline near Khvorgu, Iran (right). In the classroom activity described below, students learn first about dip and then about strike, which is opposite to the typical order for teaching the two concepts. This seems to work well, because dip is actually a more intuitive concept than is strike.

In class, students begin by viewing the inclined layers in Google Earth, as they did for the mapping homework described on the previous web page. I use the Google Earth view (below) to correlate the term "inclined/tilted" with the new term "dip", and students determine the approximate dip amount and dip direction for the contacts.

Click to enlarge.

Together, we use the Google Earth ruler tool plus Google Earth elevations to calculate the angle of slope of the flatirons (blue line below). (Note: Google Earth is actually not really adequate for determining this kind of elevation change accurately because of the lack of resolution in the elevation data set, but this particular area works well enough to make the point).
Click to enlarge.

I ask students to make a quick cross section sketch showing the topographic profile and the inclined contacts. We reason out that the slope of the flatirons isn't really an accurate measure of dip of the contacts and that we need a dip line on the contact itself, rather than on the slope.

We then generate two horizontal lines on the contact in Google Earth (red lines above right) by connecting points of equal elevation on the sides of the flatirons with a line using the Google Earth draw path tool, and we then calculate the dip of the contact using distances determined using the Google Earth measure tool. (Note: it just happens to work out in this particular spot, although in most places the results are erratic because of the limitations of the elevation data set in Google Earth.)

Click to enlarge.
Viewing the results in Google Earth 3D (left) helps students visualize their constructed dip lines and the two horizontal lines they've drawn. At this point, I finally introduce the concept of strike.

For homework, students define mappable units, locate contacts, create a geologic sketch map, add strikes and dips, and work out which unit is the oldest. They also make a cross section sketch. Being able to view the area in the Google Earth 3D view is instrumental in helping them make the cross section.

Go to the next step: Visualizing vertical contacts

Go to Visualizing inclined contacts - Visualizing strike & dip - Visualizing vertical contacts - Visualizing horizontal contacts - Visualizing folds - Other mapping projects