Scale, generalisation and quality are different properties

A chart is an organised representation rather than an exact photograph of the seabed. Three questions help explain it: at what size features were drawn; which information was selected or simplified; and what is known about the quality of the original data.

These concern scale, generalisation and evidence respectively. They share a screen or sheet, but cannot substitute for one another.

On a planar representation with constant local scale 1:N, a paper length multiplied by N gives its corresponding ground length, using compatible units. A larger denominator means a smaller scale.

A stroke needs width to be visible; that width occupies a represented area too. Ink makes a line legible, but its printed thickness declares neither an object’s physical width nor the accuracy of its position.

Cartographic generalisation can reduce a line’s vertices, group information or replace a small area with a symbol. It is more than shrinking everything in a photocopier.

The result must remain intelligible for the representation’s purpose. Consequently, visual space around a symbol cannot establish the complete extent of the feature represented.

A symbol and proportional geometry perform different functions.

Zooming may enlarge the drawing already present. Loading another dataset, compiled with different detail, is a separate operation.

They can look similar, although only the second introduces new information. Magnifying an imprecise coordinate retains its ground uncertainty; it merely changes the number of pixels occupied.

Neither a smooth curve nor a crisp digit reveals the measurement’s origin.

A sounding’s position and depth value are different quantities. The former can have a horizontal reference; the latter uses an identified vertical datum.

A well-measured depth does not establish that it was correctly located. Coverage adds another question: how much of the area was actually observed?

Detailed information at some points does not automatically turn the spaces between them into known areas.

IHO S-67 describes these dimensions of quality. This lesson does not turn categories into a clearance table: the teaching point is that scale and quality are not the same attribute.

In an invented record, “scale 1:25,000”, “survey year X”, “position evaluated by method Y” and “vertical datum Z” are complementary fields. Age may matter without independently proving a survey good or bad.

This example uses a planar grid rather than an actual coastline. A projection’s local scale can vary with location; the Mercator lesson treats that geometry.

Understanding representation and evidence differs from deciding an approach, which also involves current information, vessel and conditions.