May 04 2013

Print this Post

How to read Geological Maps

For someone who is new to Geology, reading highly technical maps could be a challenge. I remember even I had trouble reading data off of maps when I was in first year Geology class. It is nothing to be shamed of because I know even the most seasoned Geologists who read their maps wrong. But the difference between a skilled Geologist and a lay person is that a Geologist would be able to find his/her own mistake. This article will introduce you to the most fundamental aspects of Geologic maps.


True North Indicator.

True North Indicator.

Most maps are published with respect to true North. When working in the field, the compass will read the magnetic North. To correct this error, you should find the magnetic declination for the region either using the provided data on the side of the map or by extracting the information from services such as USGS official website. To take measurements off of a map, use a navigational protractor to find the angle between the feature and the North (angles are always measured clockwise). In the field, bearing of a feature is always read from the North and should be redecorated with not just the angle, but also the location, date and time. Make sure that you have adjusted your compass declination arm with the correct angle of declination before taking any measurements.

Scale and Contour Interval

Depending on the type of map, the scale may be given in several different units. This is because of the conventions we use today have been evolved slowly depending on the task. For example, most Geological maps will include a feet scale along with miles or kilometers. This will avoid the issue of having elevation and structural contours (which usually given in feet) in one unit and the map scale in another.

Scale in miles an km

Scale in miles an km

There are maps that use only one type of measurements. However, rarely you would come across a map that uses meters for contour over feet. While it does not matter which type you use, I recommend using feet since it will reduce the conversion errors in the long run.
A Geological map with contour lines.

A Geological map with contour lines.

Contour lines are always changes at a set interval, but not with a set distance.

Formations and Features

Finally, recognize the different formations and use the contacts between them to calculate strike and dip. (The key for the colour coding should be printed on the map itself.) It is a common practice to use a Navigational Protractor for measurements. If you have to draw structure contours, use two triangles to move from one line to the next. This will keep all your lines parallel to each other.

Navigational Protractor

Navigational Protractor

Some maps are published with specific information on Geology. This type of maps are based on extensive research than general geological maps. For example, a structural map of a small area could contain several faults and complex mountains. While these features will also show up on a regular geologic map, structural Geologist may find it difficult to work without having the access to specific details.

A repeated GPS based map of Aegean region between 1993 and 1998.

A repeated GPS based map of Aegean region between 1993 and 1998. The symbols are used to indicate the rotational motion of blocks.(1)

Symbols and Conventions

You may be surprised to hear that Scientists have not come to a complete agreement on what symbols we should use for what purpose. We have been using so many different symbols or variations of symbols, that sometimes reading data could be difficult. My approach to this problem is to first read all the available data on the map itself. Date of publish, the publisher, the year of publish and the location often contributes to how it is printed. For example, the following two figures shows few variations in symbols for folds.

Some of the common symbols used for folds.

Some of the common symbols used for folds(2)

Fold block separation symbols.

Fold block separation symbols(2)


The geometry of the elevation changes are often complicated, thus the patterns such as space between contour lines and their shapes can be used to identify key features. For example, the rule of V states, if several lines are pointing to one direction in the shape of a “V”, then it is most likely a valley or a fold. If it is a valley, the dip is in opposite to the point on the V. In other words, upstream is in the direction of the tip of the V.

Learn more

USGS – How to use a Compass


1. Earth and Planetary Sciences Letters, v.172, Cocard, M. andothers, Newcon-strain tsonthe rapid crustal motion of the Aegean region: recent results inferred from GPS measurements(19931998) across the West Hellenic Arc, Greece, p.3947

Permanent link to this article: http://sanuja.com/blog/how-to-read-geological-maps

1 ping

  1. Geometry of Folds » Sanuja Senanayake

    […] For more information on map reading, please read How to read Geological Maps. […]

Comments have been disabled.