Exercise 3: Reclassifying the New York State Geologic Map

Barbara and David Tewksbury, Hamilton College


Students download and merge the multiple sheets of the New York State Geologic Map together and reclassify units to create an attractive and legible version of the state geologic map at postcard scale. The class votes on the winning layout, and we have postcards printed at OvernightPrints.com *Special thanks to Brian Hynek, University of Colorado, for the basic idea for this exercise.* You might also be interested in our Full GIS course with links to all assignments.


Type and level of course
Entry level GIS course for geoscience students.

Geoscience background assumed in this assignment
Geologic time scale; conventions for naming geologic units; basics of rock types.

GIS/remote sensing skills/background assumed in this assignment
Using metadata to determine spatial reference of a data set; defining a projection; projecting a data set; creating a hillshade of a DEM; symbology and display properties; map layout.

Software required for this assignment/activity:
ArcGIS 10.6 or higher with Spatial Analyst extension (required for hillshading, but that could be left out).

Time required for students to complete the assignment:
2 homework assignments plus 2 lab sessions.


GIS/remote sensing techniques students learn in this assignment
Merging shapefiles; selecting data by four different methods (selecting by attribute, wild card selection, query language selection, and select-and-sort); reclassifying data in an attribute table; dissolving boundaries to generalize a data set in map view; clip a raster; clip a shapefile.

Other content/concepts goals for this activity
Exploring new online data sites and dealing with metadata and prep for new types of data; applying what they have learned about coordinate systems to trouble-shoot coordinate system problems with a data set.

Higher order thinking skills goals for this activity
Creation of an effective and attractive map for the data set.

Description of the activity/assignment

This is Exercise 3 in a semester-long GIS for Geoscientists course. You can find the other exercises in this series on the course summary page or by typing Tewksbury GIS Exercise into the Cutting Edge search engine.

We present students with the following scenario: "The local rock and mineral society will be holding a gem and mineral show, and one of their members has just come home from a geology conference with a handful of postcards and business cards, like the ones below, showing the geology of several places. The group would like to hand out something similar showing the geology of New York State and has asked you for help."

Exercise 3a: Introduction to the scenario described above, followed by time for students to work in pairs to decide how they would proceed, what they would need to do, and what relevant data sources they can find online. We've found that having students think this through and talk it over is a much better way to start assignments than simply handing them an assignment.

Exercise 3b: homework assignment in which students download and prep their own data. The assignment also requires some of the data that was downloaded for Exercise 2.

The New York State Geologic Map, published in the last 1960s, has been digitized and is available for download, along with a shapefile of New York State faults. Students download their own data and use what they learned in the previous exercise on coordinate systems and projections to put all data sets into the same coordinate system. This is complicated by the fact that the NYS Geologic Map files were never assigned a coordinate system by the person who digitized the sheets, and a default coordinate system of "Clarke 1866 UTM 18N" appears in the metadata. This gives us an opportunity to work out where one could go (e.g., the printed map sheets) to find the original coordinate system for the maps and to talk about the usefulness of GIS help desks, in this case to sort out what is meant by the oddball spatial reference "Clarke 1866 UTM 18N" (Clarke 1866 being an ellipsoid, not a datum).

Exercise 3c: combination in-class and homework assignment to reclassify the geologic map and prepare to design a postcard.

The New York State map sheets are hugely complicated, with hundreds of units and formation names that vary from map sheet to map sheet. This is an ideal data set to simplify by reclassification, and, after merging the sheets, students group formations by age to create maps that are legible at 5.5" x 8.5". They try out several different techniques to select data by attributes (in this case age) in order to explore how each works and to learn why it's useful to know a variety of techniques. After reclassifying, they symbolize their maps by age, generalize the age units by dissolving internal boundaries, and add a hillshade base (from SRTM DEMs that they downloaded, mosaicked, and projected). They need to clip both the hillshade raster and the faults file to the outline of the geologic map so they are ready to create a map layout for their postcards.

Ex 4: Excellence in cartography - what makes an outstanding map. Before students actually design their postcards, we take a class period to explore map design and communication via maps. We used to do this as an exercise after they had designed their postcards, but it was a bit of closing the barn door after the horses were out. Doing Ex 4 before the designing their postcards, students produce much better products. We have, however, kept it as a separate exercise so that it isn't buried in student portfolios in the middle of a geo map exercise. Students then do the map layout for their postcards.

Ex 3d and e: In class, we put each student's map up on the screen, and everyone makes comments about what works and what doesn't. This is the students' first group opportunity to address the question of what message a map conveys, and what things work and what don't. Critiquing openly in class is a very useful way of having students engage the issue, rather than just having me grade the work and hand it back. Students then vote on which postcard they think is the most effective for the scenario and complete a self-critique of their own postcards plus a workflow for the project.

Determining whether students have met the goals

Students are evaluated on the quality and accuracy of their postcard layout plus their answers to the questions on the assignment handout.
More information about assessment tools and techniques.

URLs and References

All online data sources are listed in the exercise itself or in the assignment for Exercise 2. We order printed versions of the winning postcard from OvernightPrints.com

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Exercise 3: Reclassifying the New York State Geologic Map -- Discussion  

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