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Making Geologic Maps with GIS

Article Author(s): 

Janel Day

Geographic Information System (GIS) operations are in full swing as the STATEMAP 2013 program nears completion. AZGS geologists are working to complete a total of six maps by mid-September. These maps are the culmination of a year’s worth of field data collection, data synthesis and compilation. Each map has a corresponding GIS database tailored to accommodate geologic map data. The database contains all of the spatial and descriptive information about the geology for the mapped area.

Geologic map production begins in the field. Geologists spend several months traversing pre-defined areas of Arizona’s landscape to collect geologic information about that area. Geologists collect qualitative and quantitative information in the form of observations in a field notebook or on a topographic map.

Geologists use a GPS (global positioning system receiver) to identify observation locations. At each location, the geologist records GPS waypoint information, including the waypoint identifier and geographic coordinates (latitude and longitude). In any given year, a geologist may collect 2,000+ waypoints.

GPS device used by geologists in the field.GPS device used by geologists in the field.
Observations recorded in field notebook. Observations include waypoints, qualitative and quantitative geologic information.​​Observations recorded in field notebook. Observations include waypoints, qualitative and quantitative geologic information.

Observations recorded on field sheet. Field sheet is a topographic map with annotations.Observations recorded on field sheet. Field sheet is a topographic map with annotations.

Scanned field sheet in ArcMap. Black lines represent contacts, orange area represents dacite – a volcanic rock - and labeled blue dots represent waypoints.

Scanned field sheet in ArcMap. Black lines represent contacts, orange area represents dacite – a volcanic rock - and labeled blue dots represent waypoints.

Middle Water SpringAnnotated Middle Water Spring geologic map layout.

For each waypoint, geologists record one or many observations about the physical characteristics of the rock or geologic setting. These observations are recorded in a field notebook or on a field sheet.

At the end of field season, the geologists return to the office to synthesize and compile field data into a map. This process begins by entering field data into a GIS; software used to manage and symbolize geospatial data.

To use GIS for geologic map production, field observations need to be converted to a data format that can be read by a computer. Data in the field notebook are entered into an Excel spreadsheet. These data are then imported into the geologic map, GIS database. Field sheets are scanned into the computer, producing an image file that can be georeferenced in ArcMap™, the user interface of the GIS data editing, managing and visualization environment.

In ArcMap™, geologists then digitize geologic features from the georeferenced field sheet image. The geologists begins digitizing lines that represent contacts – the surface along which two rocks meet. The geologist uses any combination of the georeferenced map, imagery and other visual data to identify these boundaries between units. These contact lines are stored in the geologic map database. Descriptive characteristics about each line are stored with that line feature in the database. From the contact lines, areas representing the geologic unit formation are created. Folds, dikes, structural observations are also added to the database.

The map compilation process continues until the geologist has created and described all features in the map area. At this point, the database is handed off to the cartographer to create the map layout for publication.

The Middle Water Spring geologic map layout was a STATEMAP 2011 deliverable. All of the information on this map is contained in a geologic map database. A subset of the geologic unit descriptions from this geologic map database is shown in the table. Arrows point from the GIS data table where geologic unit attributes are stored to the location on the geologic map layout where this information is displayed.

After the essential map elements are in place on the layout – the geologic map features, line work, description, stratigraphic column, cross section and map parameters (scale, projection, north arrow and collar information) – the map undergoes rigorous review to improve and enhance map quality. A final draft is handed off to State Geologist Lee Allison for his comments and final approval to publish. Once formally published, the map and any accompanying report are made available online at the AZGS’s Document Repository.

If the geologic map ever needs to be updated in the future, it can be easily done with GIS. With the data in a GIS database, when the map data is updated, the published map can be instantly updated.

Geoscience Information Manager
Arizona Geological Survey

 

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