The “Safety Factor”

The Safety Factor is simply a multiplier we apply to your Ground Sample Distance (GSD) to determine what contour interval you can reliably map.

1. Why we need it

GSD is the spacing between pixels on the ground (e.g., 2 cm/px). But a contour interval, say 0.5 m, has to be measurable above the noise and uncertainty in your data.

• LiDAR, photogrammetry, and other methods have some vertical error (RMSEz).

• If you tried to publish contours at exactly the vertical accuracy of your data, you’d get jittery or misleading lines.

• We multiply the GSD to set the contour interval larger than the vertical accuracy, so it’s smooth, believable, and passes quality checks.

2. How the factor comes together

A common baseline in photogrammetry:

• Vertical accuracy ≈ 2.5 × GSD (e.g., if GSD is 2 cm, vertical accuracy is about 5 cm).

• Cartographic standards (e.g., USGS, ASPRS) often say contour intervals should be at least 2× the vertical accuracy.

  Multiply those together:

  CI ≈ 2 × (2.5 × GSD) = 5 × GSD That 5× is your Safety Factor.

3. What changing the Safety Factor does

• Lower factor (3–4×) → tighter contours, but risk more noise & less reliability.

• Higher factor (6–8×) → smoother contours, more conservative, less detail.

• Field conditions matter:

  • Perfect conditions (low vegetation, great overlap, sharp optics, GCPs) might allow a lower factor.

  • Poor texture, shadows, wind, or high vegetation usually require a higher factor.

4. Example

If your GSD = 1.5 cm/pixel:

• With factor 5×: CI ≈ 7.5 cm (3 in)

• With factor 6×: CI ≈ 9 cm (3.5 in)

• With factor 3×: CI ≈ 4.5 cm (1.8 in — risky unless conditions are excellent).