Indirect measurement often relies on similar triangles. Similar triangles have the same shape but may differ in size; their corresponding angles are equal, and their sides are proportional. This property allows you to set up ratios to find unknown lengths based on known dimensions.
For example, if you want to find the height of a tree, you could measure the length of a shadow it casts and create a proportion with the height of an object of known height (like a person) and its shadow. By using similar triangles, you can calculate the height of the tree indirectly.
So, in your context, the correct term is similar.