RDM 1.1 — Accuracy Classification Explained
Survey of Kenya · RDM 1.1 Kenya 2025 · Ghilani & Wolf, Elementary Surveying 16th Ed.
What is RDM 1.1?
RDM 1.1 (Reduction and Adjustment of Observations) is the standard published by the Survey of Kenya that defines the permissible accuracy limits for geodetic and cadastral surveys in Kenya. It classifies survey accuracy into Orders and Classes based on the relationship between traverse misclosure and perimeter.
The Formula
m = C / √K
where:
m = relative accuracy expressed in mm/√km
C = linear misclosure in millimetres
K = traverse perimeter in kilometres
METARDU calculates this automatically when you run a traverse adjustment. The result tells you the accuracy class your survey achieves.
RDM 1.1 Accuracy Classes
| Order / Class | m (mm/√km) | Typical Use |
|---|---|---|
| First Order Class I | 0.5 | Primary geodetic control |
| First Order Class II | 0.7 | Secondary geodetic networks |
| Second Order Class I | 1.0 | Major cadastral surveys |
| Second Order Class II | 1.3 | Standard cadastral surveys |
| Third Order | 2.0 | Detail surveys, property subdivisions |
Worked Example
Closed traverse: 4 stations, total perimeter = 842.5 m, Linear misclosure = 0.048 m
Step 1: Convert perimeter to km
K = 842.5 m / 1000 = 0.8425 km
Step 2: Convert misclosure to mm
C = 0.048 m × 1000 = 48 mm
Step 3: Calculate m
m = C / √K = 48 / √0.8425 = 48 / 0.9179 = 52.29 mm/√km
Step 4: Compare with RDM 1.1 Table 2.4
52.29 mm/√km > 2.0 — Below Third Order
This traverse would require a better instrument, shorter legs, or more precise methods to meet cadastral standards.
Practical Implications for Cadastral Surveys
- Most land subdivision surveys in Kenya should meet Third Order (m ≤ 2.0 mm/√km) minimum.
- Town surveys and dense urban subdivisions typically require Second Order Class II.
- Precision ratio alone (e.g. 1:10,000) is not sufficient — the RDM 1.1 formula must be applied to determine the accuracy class.
- METARDU displays the accuracy badge on every traverse result, so you can immediately see if your survey meets the required standard.
Reference
- Survey of Kenya. RDM 1.1 — Reduction and Adjustment of Observations. 2025 Edition. Survey of Kenya, Nairobi.
- Ghilani, C.D. & Wolf, P.R. (2022). Elementary Surveying: An Introduction to Geomatics, 16th Ed. Pearson. Chapter 12.
- Republic of Kenya. Survey Act, Cap 299. Government Printer, Nairobi.