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January 13, 2008

A map or relief model, to be most useful, must accurately show locations, distances and elevations on a given base of convenient size. This means that everything featured on the map or model (land area, distances, rivers, lakes, roads, and so on) must be shown proportionately to its actual size. The proportion chosen for a particular map is its scale.

The scale of a map can be defined simply as the relationship between distance on the map
and the distance on the ground, expressed as a proportion, or representative ratio.

This "representative ratio" means that 1 cm on a map is equivalent to

Why do we need to adjust the planimetric scale?

The smaller the scale of a map is, the fewer the features that can be accommodated. Obviously, therefore, the larger the scale the more comprehensive the map and of more use to the planning process. Considering that participatory 3-D models aim at providing a visual aid capturing the details of the territory, the larger the scale the better.

The choice of the scale and hence the size of the model should take account of the need for accuracy as well as the need for enough space in which physically to construct and store the model. The ideal scale for 3-D modeling is 1:10,000 or larger. If your reference map is at 1:50,000-scale, it needs to be re-scaled to 1:10,000 to make it suitable for 3-D modeling. At 1:10,000, 1 centimeter on the model corresponds to 100 meters on the ground - a pretty comfortable scale for people to pin-point salient features.

The table above demonstrates the simple arithmetic of re-scaling smaller scale maps (1:50,000, for example), to more detailed larger scale maps (1:10,000), and the relationships between the physical dimensions of the model to the geographical area represented by the same.

A number of factors influence the options for re-scaling. First you have to identify and measure the area you want to reproduce. For your ease, you will select a rectangular shape including the core area (e.g. protected area, watershed, ancestral domain, or other) and its environs of ecological, cultural and economic significance.

If the core of a protected area is a mountain, the rectangle will include the downhill catchments and possibly the settlement areas where most dependent communities reside.

If the core is a lake or coastal area, all catchments draining into the main water body should be represented.

This is important for the analytical process, which usually follows the construction of the model and whence all those referring to the model do assess causes and effects.

Having defined the area of interest, the next step is to decide on the scale in which it is to be reproduced. The scale should permit the desired level of detail on a model that is manageably sized. At a 1:10,000 scale, households and other point-form features can be individually located. A 1:20,000-scale model accommodates less information, but settlement details can still be represented.

The physical size of the model needs to be seriously considered in view of the space needed both for display, and for storage. Generally, models are constructed and stored at the same location. The question of dimension therefore, should be defined beforehand with the prospective caretaker, which could be the local government, a school, a people's organization or other. Last but by no means least, the larger the model, the more time is needed for its manufacture and the more resources (human and financial) will need to be mobilized.

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