On the Edge: Mapping the Delta

By Tracy Cozzens

Surveyors install and configure a base and rover for a 13,000-hectare survey of the Plains Kogoni in Mali.

In the heart of landlocked Mali, between the Atlantic Ocean 800 miles to the south and the Sahara desert to the north, lays the extraordinary Inner Niger River Delta, also known as the Macina, a 1.8 million hectare oasis of lakes and floodplains with a vast potential for hydro agriculture.

CIRA, a major West African consulting engineering firm, working on behalf of the Office du Niger, a quasi-governmental Mali company charged with managing more than100,000 hectares of irrigated delta land, has completed surveying an additional 25,000 hectares for hydro-agriculture development.

Created in 1991, CIRA is an engineering and applied research consulting firm working in transportation, hydraulics, civil engineering and the environment. Based in Bamako, Mali, the firm works in more than 15 African countries, primarily in West Africa, Central Africa and East Africa.

In the course of two months during the dry season, two CIRA survey teams, each equipped with three Spectra Precision ProMark 500s, a base station, and two rovers connected via UHF, completed the entire 25,000 hectare survey collecting four points in x, y, and z per hectare to produce a digital model. The model enabled the production of rough pre-study with all plans and a detailed pre-project CAD drawings for drainage, irrigation canals, and related infrastructures.

A very short eight-month contractual time set to complete the different studies meant that the land survey study would have to be completed as quickly as possible. The first thought was to use aerial photography combined with LIDAR, but setting this up would have taken too long, according to a CIRA spokesperson. Instead, CIRCA chose to employ differential GNSS, using base and rovers working in real-time kinematic. CIRA’s experience suggested the firm would achieve reliable results much quicker using only optical total stations. CIRA elected to use Ashtech ProMark 500 GNSS receivers for the project. From experience, they knew the models were easy to set up and use, lightweight, offered long battery life in the field, and field to office data transfer would be easy. Their expectations were met, and the job was completed within two months and on time.

The ProMark 500 RTK survey system provides short time to fix, long-range RTK and solution reliability. Its BLADE technology provides multi-constellation signal processing with the use of SBAS and GLONASS ranging signals to strenghten the GPS solution.

Trimble acquired Ashtech in 2011, making it part of Spectra Precision.

Setting up bitter points for calibration of satellite images on the corridor Sarh – Abeche in Chad (800km).

Reference station during the survey topo Richard Toll road – N Dioum (120 miles) in Senegal.

A reference station during the survey topo Zégoua Sikasso road (95 km) in Mali.