Understanding map scales

When zooming around the map, we could notice the Scale changing in the status bar. GIS software (apart from web mapping solutions) usually use scales instead of zoom levels. The map scale is an important concept of cartography, and its use was inherited by GIS software. The scale shows the ratio (or representative fraction) between the map and the real world. It is a mapping between two physical units:

For example, a Scale of 1:250,000 means 1 centimeter on the map is 2500 meters (250,000 centimeters) in the real world. However, as the map scale is unitless, it also means 1 inch on the map is 250,000 inches in the real world, and so on. With the scale of the map, we can make explicit statements about its coverage and implicit statements about its accuracy. Large scale maps (for example, with a scale of 1:10,000) cover smaller areas with greater accuracy than medium scale maps (for example, with a scale of 1:500,000), which cover smaller areas with greater accuracy than small scale maps (for example, with a scale of 1:1,000,000).

We can easily imagine scales on paper maps, although the rule is the same as on the map canvas. On a 1:250,000 map, one centimeter on our computer screens equals 2500 meters in the real world. To calculate this value, GIS software use the DPI (dots per inch) value of our screens to produce accurate ratios.

By using scales instead of fixed zoom levels, GIS software offers a great amount of flexibility. For example, we can arbitrarily change the Scale value on our status bar and QGIS automatically jumps to that given scale. The definition of map scale will follow us along during our work as QGIS (like most of the GIS software) uses scale definitions in every zoom-related problem. Let's see one of them--the scale dependent display. We can set the minimum and maximum scales for every layer and QGIS won't render them out of those bounds. Let's right-click on one of the layers and select Properties. Under the General tab, we can check Scale dependent visibility. After that, we can provide bounds to that layer. By providing a minimum value of 1:500,000 to the layer and leaving the maximum value unbounded (0), we can see the layer disappearing on 1:500,001 and smaller scales: