Understanding Map Scale

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What is the definition of scale? Scale is the relationship that the depicted feature on map has to its actual size in the real word (more: map scale).  

All maps are modeled representations of the real world and therefore the features are reduced in size when mapped.  

In other words, scale is the measurement of the amount of reduction a mapped feature has to its actual counterpart on the ground.

Representing Scale on a Map

All maps will have an indicator of the scale of the map.  The purpose of a scale is to show the relationship of the map distance to the ground distance. In other words, how much does a distance measured on the map represent in the real world?

Map scale units will most often be in one of two measurements systems. The map scale may show metric units such as meters or kilometers. The other common measurement system, particularly in the United States, is to show the map scale in a British imperial unit and United States customary unit of distance such as feet or miles.

A map that doesn’t conform to a specific scale will be indicated by the words “not to scale” (or NTS).  This notations is most commonly found on graphic style maps such as the  “we are here” or “how to get here” style maps used on invitations.  

Since GIS relies on a minimum threshold of accuracy and precision, almost all GIS-based maps will have a scale.

What are the Three Ways to Show Scale on a Map?

There are three ways to show the scale of a map: graphic (or bar), verbal, and representative fraction.  

Graphic Map Scale

Graphic scales, also know as bar scales, as indicated by the name, show the scale graphically.

Some maps will have bars to denote the map units, other graphic map scales will have tick marks to show units.

A black and white graphic showing a map scale in kilometers with tick marks and a map scale in miles with bar scales.
Graphic map scales can either show tick marks (top) or scale bars (bottom). Graphic: USGS, public domain.

Some graphic map scales will show both metric and imperial/customary units for reference like the map bar scale below.

Example of a bar scale used on a map.
Bar scale showing graphically the ratio of map units to ground units. The top scale shows the comparable ground measurement in kilometers (km) and the bottom bar scale shows the comparable length in miles.

Verbal Scale on a Map

A verbal scale is text based, with the scale shown as a number and type of unit measurement equal to a specified unit measurement on the ground.  The left side of the verbal is the unit of measurement on the map and the right side of the ratio is the unit measurement on the ground.  

For example the verbal scale, 1″ = 100′ means that one inch measured the the map represented 100 feet on the ground.  This type of scale is sometimes confused with Representative Fraction scales.

Representative Fraction as a Scale on a Map

Like verbal scale, representative fraction (RF) scale is also a text based scale but no units are shown.  

For RF, scale is a simple ratio of map to ground measurement with a colon between the two measurements .  

For example, a RF scale of 1 : 1,200 means that every one unit on the map is equal to 1,200 units on the ground.  There is no notation of the actual unit type used on a RF scale.  

Therefore a RF scale of 1:1,200 is the same scale as a verbal scale of 1″ = 100′.

A cropped view of a map for Oklahoma that has verbal map scale on it.
An example of verbal scale on a map. Base map of the state of Oklahoma prepared by United States. Bureau of the Census. Geography Division. Library of Congress, public domain.

Large versus Small Scale Maps

Maps are described as either large scale or small scale.  

Large scale maps

Large scale maps show a smaller amount of area with a greater amount of detail.  The geographic extent shown on a large scale map is small.  

A large scaled map expressed as a representative scale would have a smaller number to the right of the ratio.  For example, a large scale map could have a RF scale of 1 : 1,000.  Large scale maps are typically used to show neighborhoods, a localize area, small towns, etc.

A New York City planning map with a person using a black sharpie to write on it.
This planning map created by the GIS department for New York City’s bike share program is a large scale map.

Small scale maps

Small scale maps show a larger geographic area with few details on them. Small scale maps show large areas like countries or huge geographic areas such as continents. 

The RF scale of a small scale map would have a much larger number to the right of the colon such as 1 : 1,000,000.  Small scale maps are used to show the extent of an entire country, region, or continent.

A shaded orange map showing areas of the United States with higher than normal carbon dioxide in the atmosphere.
This map of atmospheric carbon dioxide anomalies within the United States would be considered a small scale map as it represents a large geographic area. Map NASA, public domain.

How Scale Affects Feature Representation

The larger the scale of the map, the better the features that can be detailed.

A map that shows the water network of a small area may show the river as a polygon layer and will show the tributaries of that river.  A small scale map covering the area would show that same river as a line feature and the tributaries would be removed (a process known as generalization).  

The smaller the scale of the map, the less the actual detail of a feature is preserved.  Smaller scale maps have river features that have the lines smoothed out whereas a large scale map would show more detail about the twists and turns of that same river.

Why Generalization is Related to Map Scale

So why not show the same level of detail regardless of the map scale?  There are two main reasons.  

The first reason is the noise level.  Showing a lot of detail for features on a small scale map would cause a lot of confusion on the map.  

By reducing the amount of detail to show only the most important aspects of a feature, the map shows a clearer picture of the area.  

The second reason is the file size.  

Features that have a lot of detail are larger in file size.  For a small scale map, loading several large layers will slow down the map production.

An Example of Small Scale Versus Large Scale Representation

Marina del Rey is an area of Los Angeles County that has one of the largest man-made small boat harbors in the United States.

Aerial of Marina del Rey

The representation of this harbor on maps of the area is dependent on the scale of the layer used.  

The layer showing the counties for the entire United States shown in the image below has a very generalized coast line for this area.  The is almost no detail in the coastline and the harbor is not represented at all. (Related: GIS Data and the Coastline Paradox)

Marina del Rey area on a small scale map.
Marina del Rey area on a small scale map. The harbor is not shown at all on the map.

The map below shows the same coastline with a layer of all counties for the state of California.  While still a small scale layer, the coastline shows more detail.  The Marina del Rey harbor is represented by a small inlet on the map.

Marina del Rey area on a county map of the state of California.
Marina del Rey area on a county map of the state of California.

In a large scale layer created to show just the County of Los Angeles boundary, the coastline for this area contains the highest level of detail, and a recognizable harbor is represented.

Marina del Rey at the county level.
Marina del Rey at the county level.

The examples of how the detail of a coastline changes depending on the scale of the layer helps to illustrate as well the importance of carefully considering the scale of any data used for mapping and spatial analysis.  

Small scale data inherently is less accurate and less detailed than large scale data.  

Using small scale data for large scale analysis can lead to gross errors.  Data created for small scale purposes should not be used in large scale maps.  

Large scale data unless generalized, should not be used in small scale maps.

This article was originally published on September 1, 2011 and has since been updated.

Watch: Large Scale Versus Small Scale Maps

Related Resources

What’s in a Map? – Understand the basic elements that make up a well formed map.

Cartographic Resources – Resources for learning more about cartography and cartographic techniques.

 

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