Basic GIS Tools - Map Scale

A map's scale is the ratio between units of measurement on the map and units of measurement in the real world.  Simply put, a map scale of 1:24,000 means 1 inch on the map equals 24,000 inches in the real world.  ArcMap shows the current scale in the 'Standard' toolbar:


The scale of this map 1:307,248.  This can be expressed in the following ways:
   - absolute scale of 1:307,248
   - relative scale of 1 inch = 307,248 inches = 25,604 feet = 4.85 miles (etc.)

It is very easy to change a map's scale in ArcMap, in the following ways:
   - simply zooming in or out will change the scale, which will be reflected in the 'Standard' toolbar
   - in the 'Standard' toolbar, a drop-down list shows several predefined scales to choose from
   - simply type in a new scale in the 'Standard' toolbar

The scale of the 'Data View' in ArcMap will likely be different than the scale of the 'Layout View' in ArcMap.  The reason is that the former is based on the scale computed from what's seen on your computer screen, while the latter is based on the size of the paper specified for the map.  You will see that the scale changes when you switch between 'Layout View' and 'Data View' (this will be discussed more in an upcoming section).

The scale in the 'Data View' is the scale based on your computer screen, and is based on a 15-inch screen.  Most MEDEP users with Dell PCs and flat monitors have 15" screens.  If that is the case, then a scale of 1:2400 means an inch on your computer screen is 2400 inches in the real world.  If you do not have a 15" screen, then the scale will be off a bit.

The scale in the 'Layout View' is based on the page size specified and the size of the data frame on that page.  It is a simple ratio of a single unit of measure on the page compared to the real world.  1:2400 here means that 1 inch on your printed map equals 2400 inches in the real world.

Large vs. small scale
One of the most confusing concepts to new GIS users are those of 'large' scale and 'small' scale maps.  These terms in mapping run counter to how they would be applied elsewhere.  A large scale map is actually a large map of a small area (with a scale like 1/1000), while a small scale map is going to be a small map of a large area (with a scale like 1/500000).  Since the terms refer to the scale, which is a fraction, the larger scale in the two examples above is 1/1000.  If you want a map of the whole state on 11x17, you want a small scale map.