20 January 2016

Going digital: Building a better geological map of Alaska

Posted by nbompey

By Rebecca Fowler

This is part of a new series of posts that highlight the importance of Earth and space science data and its contributions to society. Posts in this series showcase data facilities and data scientists; explain how Earth and space science data is collected, managed and used; explore what this data tells us about the planet; and delve into the challenges and issues involved in managing and using data. This series is intended to demystify Earth and space science data, and share how this data shapes our understanding of the world.

The Alaska Geological Map shows the generalized geology of the state. Each color represents a different type or age of rock. Credit: USGS

The Alaska Geological Map shows the generalized geology of the state. Each color represents a different type or age of rock.
Credit: USGS

In the early 1900s, before Alaska was part of the United States, geologists roamed this northern territory on foot and horseback, noting its features and terrain on hand-drawn maps. Nearly 100 years later in 1996, U.S. Geological Survey (USGS) research geologist Frederic Wilson and a dozen colleagues undertook the task of using some of the information contained in these field notes, sketches and maps, along with many other sources of data, to create the first fully digitalized geological map of Alaska.

There are a several lengthy steps involved in making a digital geological map. Geologic and geographic data must be compiled, in this case from some sources dating back to pre-digital times. The published 1:250,000-scale (one inch equals about four miles) geologic quadrangle maps of Alaska, of which there are around 100 for the 153 total state quadrangles, were the primary sources used to create the map. The scientists were also able to locate unpublished compilations of data for a few more quadrangle maps. For the remainder of the state, and to update most of the published maps, any available source was used. “These included 1:63,360-scale maps [one inch equals one mile] published by the USGS and the Alaska Division of Geological and Geophysical Surveys, journal articles, original field notes, air photo interpretation, Google Earth and actual fieldwork by me and others,” said Wilson.

The next step in making a digital map is engineering and combining the data. Wilson explained that, due to their age, the vast majority of his data had to be digitized and their map unit descriptions entered into the database. Upon capturing the map linework digitally and entering the map unit descriptions, or geologic formations, into the databases, the map units were then categorized and correlated from map to map, reducing the more than 15,000 original source map units to about 1,350 units. For each of these units, the maximum and minimum age, lithologic characteristics and geologic setting were added to related database tables.

“Concurrently, we constructed databases of radiometric age determinations, more than 7,000 total,” said Wilson. “As the process continued, a series of regional maps were released and the compiled unit descriptions from each of these were also added to another related database.”  

Ultimately, the scientists created more than a dozen interrelated databases. When the time came to create the state map, these databases were used to help Wilson group like units, which reduced those displayed on the digital version of the map to about 450 databases. The regional map unit descriptions, as well as the original source map unit descriptions were then used to write the composite unit descriptions for the state map. The result of this 20-year effort is actually a database from which many derivative maps can be created. The final product displays the digitized images at a scale of about 25 miles per inch on a full-size printed map; a 204-page document and supporting databases provide explanatory details and color codes for the different types of rock found in Alaska.

Among the challenges encountered by Wilson when creating the map: deciphering notes from geologists past. “My sources spanned more than 100 years of data collection by a large number of geologists,” said Wilson. “Each of us as geologists has a particular perspective, influenced by our experiences and the paradigms of our time. One of my big challenges was trying to get in each geologist’s head, to try to understand what they saw; what their information was telling me; what clues in the data collected from 50 to 100 years ago can I understand and utilize in a modern plate tectonic paradigm.” Wilson explained that an early map description that simply reported ‘sandstone and shale’ is now understood to refer to the turbiditesa type of sedimentary rock composed of layered particles that grade upward from coarser to finer sizes—of the Kuskokwim Group in west-central Alaska.

The map not only reveals new knowledge about Alaska’s geology, but the value that can lie hidden in geological collections. “I have long understood how little we know about the geology of the state compared to the conterminous United States. At the same time, I was surprised how much we do know, squirreled away in forgotten or obscure sources.”

Potential map users include scientists, land management agencies such as the National Park Service, and those in the mining and energy industries. The combination of broad and detailed information provided in the map allows users to search for patterns and trends that are otherwise not be apparent. “The digital nature of the data enhances their ability to do this and the digital nature of the product also makes it much easier for users to incorporate other datasets as desired,” said Wilson.

The map and its related products are free to use—explore them here.  

— Rebecca Fowler is a science communicator and the Director of Communications and Outreach at the Federation of Earth Science Information Partners (ESIP).