GNSS Surveying in Sierra Leone: Sierra Leone 1924 Datum Guide
GNSS RTK surveying in Sierra Leone has historically relied on the Sierra Leone 1924 datum (also called Sierra Leone Colony 1924), based on the War Office ellipsoid — the same ellipsoid family used across several former British colonial territories in West and Southern Africa. Two historical projected grids were defined for the Freetown Peninsula area (EPSG:2159 and EPSG:2160), though modern GNSS surveying increasingly uses WGS84 with UTM Zone 28N or 29N depending on location. Sierra Leone does not currently have a public national CORS network, making Base + Rover the practical standard for most field survey work, as demonstrated in a recent APEKS AM02i cadastral survey deployment in the country.
- 1. Sierra Leone Survey Overview
- 2. Geodetic Datum: Sierra Leone 1924 and the War Office Ellipsoid
- 3. Coordinate Systems: Historical Grids and Modern UTM Zones
- 4. No National CORS: Why Base+Rover Is Standard
- 5. Step-by-Step RTK Setup for Sierra Leone
- 6. Key Survey Applications
- 7. Field Case: AM02i Total Station Deployment
- 8. APEKS Equipment for Sierra Leone Projects
- 9. FAQ
1. Sierra Leone Survey Overview
Located on the southwest coast of West Africa, Sierra Leone covers an area of 73,252 square kilometers with a 402-kilometer coastline along the Atlantic Ocean. Its topography transitions from low-lying coastal mangrove swamps and tidal creeks to an undulating interior plateau, rising to mountainous terrain in the east along the Loma Mountains, crowned by Mount Bintumani at 1,948 meters.
Geospatial surveying demand in Sierra Leone is driven by four primary sectors:
Cadastral Surveying & Land Tenure Reform: Securing property rights and digitizing land registries in both urban municipalities and rural chiefdoms.
Mineral Resource Concessions: Delineating and monitoring extraction leases for diamonds, rutile (titanium ore), bauxite, iron ore, and gold deposits across the Southern and Eastern Provinces.
Civil Infrastructure Development: Providing baseline ground control for transport corridors, highway rehabilitations, and port expansion projects.
Urban Expansion of Greater Freetown: Mapping informal settlements, infrastructure right-of-ways, and steep coastal hillside parcels across the Western Area Peninsula.
2. Geodetic Datum: Sierra Leone 1924 and the War Office Ellipsoid
The historical geodetic framework of Sierra Leone is defined by the Sierra Leone 1924 datum (formally referenced in cartographic literature as the Sierra Leone Colony 1924 datum). Established under British colonial administration, the datum was calculated on the War Office ellipsoid, characterized by a semi-major axis $a = 6,378,300\text{ m}$ and an inverse flattening $1/f = 296$.
The War Office ellipsoid served as a standard geodetic figure across British imperial mapping programs in Africa, shared by historical geodetic frameworks such as the War Office datum in Ghana. The fundamental geodetic origin for Sierra Leone Colony 1924 was established in the Freetown Peninsula area, from which primary triangulation networks were subsequently propagated eastward across the protectorate.
While modern engineering projects, aerial photogrammetry, and satellite positioning have shifted almost entirely to the WGS84 ellipsoid, legacy land registry grants, historical mining concession certificates, and colonial boundary demarcations remain tied to the 1924 framework. Accurately integrating these legacy records with modern GNSS RTK observations requires localized coordinate transformation parameters.
3. Coordinate Systems: Historical Grids and Modern UTM Zones
Surveyors working in Sierra Leone encounter two distinct cartographic frameworks: historical local grid projections developed specifically for the Colony area around Freetown, and the modern global Universal Transverse Mercator (UTM) projection on WGS84.
| Coordinate System | EPSG Code | Projection / Ellipsoid | Coverage & Usage Notes |
|---|---|---|---|
| Sierra Leone 1924 / New Colony Grid | EPSG:2159 | Transverse Mercator / War Office | Historical cadastral grid covering the Freetown Peninsula and Western Area. |
| Sierra Leone 1924 / New War Office Grid | EPSG:2160 | Transverse Mercator / War Office | Historical triangulation grid covering legacy colonial surveys and military charts. |
| WGS 84 / UTM Zone 28N | EPSG:32628 | Transverse Mercator / WGS 84 | Standard modern projection for Western Sierra Leone (west of 12°W), including Freetown. |
| WGS 84 / UTM Zone 29N | EPSG:32629 | Transverse Mercator / WGS 84 | Standard modern projection for Eastern Sierra Leone (east of 12°W), including Koidu and Kenema. |
For new engineering, topographic, or mining survey campaigns, teams should configure field controllers to WGS84 UTM Zone 28N or Zone 29N. The central meridian separating Zone 28N and Zone 29N lies along the 12°W meridian, which runs directly through central Sierra Leone (roughly bisecting Makeni). Historical grids (EPSG:2159 and EPSG:2160) should be reserved for transforming archived boundary deeds and legacy concession records.
4. No National CORS: Why Base+Rover Is Standard
Symptom: Surveyors attempting to configure Network RTK (NTRIP) rovers discover no publicly accessible, nationwide continuous operating reference station (CORS) streams across Sierra Leone.
Cause: While academic institutions and private mining consortia occasionally operate localized GNSS reference receivers for proprietary monitoring, there is currently no government-administered, publicly available CORS infrastructure providing real-time differential corrections across the territory.
Fix: Deploy an autonomous local Base + Rover kit. Using a high-power standalone base station — such as the APEKS MAX5 equipped with an internal 5W LoRa radio — delivers reliable UHF/LoRa correction coverage over distances up to 25 km. This eliminates dependence on cellular internet, which remains unreliable in rural provincial districts and remote mining concessions.
5. Step-by-Step RTK Setup for Sierra Leone
Confirm whether project deliverables require modern WGS84 UTM coordinates or conversion into the legacy Sierra Leone 1924 Colony Grid. Modern civil engineering works, road designs, and topographic surveys use UTM Zone 28N or 29N. Historical boundary resolutions may require calibration to archive deeds.
Set up a dedicated base station (such as the MAX5 or an AP-series GNSS receiver) on a sturdy tripod over a verified geodetic pillar or established site benchmark. If occupying an arbitrary point, initiate a long static observation session to compute an accurate autonomous position, or tie into known benchmarks using post-processing.
In field data software such as ApekSurv, create a new project and select the WGS84 datum. Choose UTM Zone 28N (for sites west of 12°W) or UTM Zone 29N (for sites east of 12°W). If integrating legacy 1924 grid data, enter localized datum transformation parameters or perform a multi-point site calibration against verified boundary markers.
Configure the base station internal radio (or external LoRa link) to broadcast standard RTCM 3.2 corrections. Set the rover receiver to match the radio frequency and protocol. Monitor satellite tracking and PDOP values until the receiver confirms an RTK Fixed positioning status with centimeter-level precision.
Prior to executing boundary stakeout or topographic pickup, occupy a secondary known control point with the rover. Verify that measured horizontal and vertical coordinates match benchmark records within required survey tolerances (typically ±15mm to ±25mm).
6. Key Survey Applications
Field crews in Sierra Leone routinely deploy positioning hardware across several critical operational environments:
- Cadastral Boundary Delineation & Land Titling: Formalizing legal parcel lines for individual properties, commercial subdivisions, and municipal grants in urban districts like Freetown, Bo, and Kenema.
- Mining Lease & Concession Management: Establishing precise boundary corners for diamond concessions in Kono, rutile extraction tracts in Moyamba and Bonthe, and bauxite operations in Port Loko.
- Infrastructure & Transport Corridor Engineering: Providing continuous topographic ground profiles and structural alignment control for rural highway expansion and drainage networks.
- Municipal Mapping & Hazard Management: Capturing elevation models across steep peninsular slopes to monitor landslide susceptibility and regulate informal urban development around Freetown.
7. Field Case: AM02i Total Station Deployment
"A survey team in Sierra Leone deployed the APEKS AM02i Android total station for cadastral field work, combining prism-based measurement with the device's Android 14 interface for on-site data handling."
👉 Read the complete field case study: APEKS AM02i in Sierra Leone: Android Total Station Field Case
8. APEKS Equipment for Sierra Leone Projects
Navigating Sierra Leone's humid tropical coastal climate, heavy rainfall seasons, and remote interior concessions requires ruggedized hardware with flexible correction modes.
| Model | Key Hardware Capabilities | Primary Sierra Leone Application |
|---|---|---|
| AP20 AR | 120° IMU tilt compensation, dual AR visual stakeout cameras, 1408 channels | Standard cadastral boundary stakeout and urban parcel mapping |
| AP40 Laser+ | Green laser rangefinder (±8mm + 5mm/m within 30°), 120° IMU tilt, IP67 ruggedness | Mining concession boundaries, steep pit walls, road corridor offsets |
| MAX5 Base | 5W LoRa radio, 25km transmission range, OLED display, 13,600mAh internal battery | Autonomous local base station for remote concessions lacking CORS coverage |
| AM02i | Android 14 OS, 2" angular accuracy, 1000m reflectorless EDM, dual hot-swap batteries | Legal cadastral boundary measurement and optical control (field-verified) |
| APS1 | UM980 GNSS chipset, Galileo HAS / BDS PPP-B2b autonomous positioning (~$450) | Rapid mineral prospect reconnaissance, GIS inventory, asset mapping |
9. FAQ
What datum does Sierra Leone use for land surveying?
Historically, land surveys in Sierra Leone were conducted on the Sierra Leone 1924 datum (Sierra Leone Colony 1924) based on the War Office ellipsoid. While modern GNSS operations predominantly use WGS84 with UTM projections, the 1924 datum remains legally relevant when referencing archived property deeds and colonial survey records.
What is the War Office ellipsoid and where else is it used?
The War Office ellipsoid is a historical geodetic reference surface with a semi-major axis of 6,378,300 meters and an inverse flattening of 1/296. It was widely deployed across British colonial mapping programs in Africa, notably serving as the basis for national historical datums in Sierra Leone, Ghana, and Nigeria.
Does Sierra Leone have a national CORS network for RTK surveying?
No, Sierra Leone does not currently have a publicly accessible national CORS network. Surveyors achieve centimeter-level RTK positioning by deploying autonomous local Base + Rover kits, utilizing high-power LoRa or UHF radio links such as the APEKS MAX5 base station.
Which UTM zone should I use for surveying in Sierra Leone?
Sierra Leone spans two UTM zones divided by the 12°W meridian: UTM Zone 28N (EPSG:32628) covers western areas, including Freetown and Port Loko; UTM Zone 29N (EPSG:32629) covers eastern territories, including Bo, Kenema, and Koidu.
SIERRA LEONE 1924 READY. FIELD-VERIFIED. NO CORS DEPENDENCY.
APEKS RTK receivers support WGS84/Sierra Leone 1924 transformation and independent Base+Rover deployment for cadastral and mining projects — field-verified with the AM02i in Sierra Leone. IP67/IK08. 120° IMU.
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