Contours and elevation accuracy – interpolation limit from a contour map The accuracy with which the elevation of points can be obtained from a contour map (by interpolation) is typically limited to approximately what fraction of the contour interval?

Introduction / Context:
A contour map represents ground elevations at discrete contour lines separated by the contour interval (CI). When estimating the elevation of a point between contours, we interpolate. However, the result cannot be more accurate than a certain fraction of the contour interval because of line spacing, drawing limits, and terrain irregularity. This question asks for the commonly accepted limit used in practice and in examinations.

Given Data / Assumptions:

• Contours are correctly located with typical planimetric accuracy.
• Terrain between contours is not pathologically irregular at sub-contour scale.
• We refer to routine map-reading, not special photogrammetric refinement.

Concept / Approach:

Interpolation visually or by proportional distance assumes linear variation of slope between adjacent contours. Even under good mapping practice, the practical accuracy of such interpolated spot heights is limited. A widely used rule-of-thumb is that elevations from contours are reliable only to about one-half of the contour interval. Finer claims (one-third, one-fourth) are optimistic unless supported by additional data (spot levels, profiles) and high mapping precision.

Step-by-Step Solution:

Verification / Alternative check:

Surveying texts and mapping practice guides commonly caution that spot levels from contours should be treated with an uncertainty on the order of half the contour interval unless more precise control exists.

Why Other Options Are Wrong:

1/3, 1/4, 1/5 of CI suggest tighter accuracy than generally justifiable for standard contour maps without dense control.

Common Pitfalls:

Overstating precision from neatly drawn maps; ignoring cartographic generalization; assuming uniform slope between widely spaced contours.

Final Answer:

1/2 of the contour interval