What determines the scale of a map in GIS?

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What determines the scale of a map in a Geographical Information System? There is a fantastic post about cartographic scale on the National Geographic site. The article makes the point that as the scale of a map changes then the cartographer is faced with choices. When producing smaller scale maps (think ship-in-a-bottle) that cover large areas, map making becomes difficult because things need to be left out. But on what basis does a cartographer choose which to leave in and which to leave out? Usually it is with the “audience” in mind – walking maps will highlight features important to walkers, while street maps will highlight features important to road users. This all makes sense.

But what happens to the scale of a map when it finds its way into a Geographical Information System? More often than not the cartographer’s art gets lost and maps find themselves being used for purposes and at scales other than those they were intended for. This can be a trap for inexperienced map users.

All maps are simplifications (models) of reality. How much information is contained in a map depends on the scale of the map. On a paper map…

  • a scale of 1:10,000 means that one length of measurement equals ten thousand in real life.
  • a 1:250,000 (small) scale map will contain less information than a 1:10,000 (large) scale map, and boundaries usually have less detail.

It is important to understand that cartographers create maps with a scale of printing and interpretation in mind. The scale of a map relates very closely to the amount of time and effort that went into producing it. A map produced at the end of a week in the field will be of poor quality compared to a map produced by the same people over the same area following a year of fieldwork. A mistake that people sometimes make is to enlarge a map on a photocopier in the hope of producing more detailed mapping. Sometimes it is OK to do this, but increased size does not equate to increased detail – no matter how many times you enlarge a world atlas on a photocopier, local streets do not appear!

It is important that a GIS analyst understands scale because many GIS maps have their origins in old paper maps. During the 1990s governments around the world put a lot of effort into building their digital map infrastructure, and they converted (digitized) their paper map archives into maps that could be used in GIS software. However, in a GIS the cartographic “tells” of scale that existed on the original paper map (thick lines, scalebars, etc.) disappear, so the map effectively becomes scaleless.

For maps that are Surveyed by professional surveyors such as property boundaries and roads, this issue is less important than for maps that are Interpreted (ie. environmental themes and social surveys).

The take-home message is that whenever possible, the maps you use for your projects should be at an appropriate scale for the project’s purpose. Because the scale of a map relates to the scale of its collection, and not the scale of its display, whenever appropriate, on your printouts you should indicate the scale of the original mapping as well as your GIS’s auto-generated scale bar.

Reference:

The National Geographic article is here: Where Scale Permits http://t.co/fkz27xQz

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