Operational sampling designs for poorly accessible areas based on a multi-objective optimization method

Highlights: • Estimating sampling fieldwork time by mapping operational constraints is possible. • MOOS reduces fieldwork time compared to other methods. • It allows a practitioner to choose among a variety of sampling designs. Abstract: Sampling for Digital Soil Mapping is an expensive and time-constrained operation. It is crucial to consider these limitations in practical situations, particularly when dealing with large-scale areas that are remote and poorly accessible. To address this issue, several authors have proposed methods based on cost constraints optimization to reduce the travel time between sampling sites. These methods focused on optimizing the access cost associated to each sample site, but have not explicitly addressed field work time required for the whole sampling campaign. Hence, an estimation of fieldwork time is of great interest to assists soil surveyors in efficiently planning and executing optimized field surveys. The goal of this study is to propose, implement and test a new method named Multi-Objective Operational Sampling (MOOS), to minimize sampling route time, while ensuring that sample representativeness of the area is maintained. It offers multiple optimal sampling designs, allowing practitioners to select the most suitable option based on their desired sample quality and available time resources. The proposed sampling method is derived from conditioned Latin Hypercube sampling (cLHS) that optimizes both total field work time (travel time and on-site sampling time) and sample representativeness of the study area (cLHS objective function). The use of a multi-objective optimization algorithm (NSGA II) provides a variety of optimal sampling designs with varying sample size. The sampling route time computation is based on an access cost map derived from remote sensing images and expert annotation data. A least-cost algorithm is used to create a time matrix allowing precise evaluation of the time required to connect each pair of sites and thus determine an optimal path. The proposed method has been implemented and tested on sampling for pH H2O mapping within a 651 points kilometric grid in the northern part of Saudi Arabia, where soil analyses were conducted over a 1,069 km2 area. MOOS method was compared to two other common approaches: classical cLHS and cLHS incorporating access cost. The performance of each method was assessed with the cross-validated RMSE and sampling route time in days. Results show that the MOOS method outperforms the two others in terms of sampling route time, especially with increasing sample size, gaining up to 1 day of work for the presented case study. It still ensures a relevant map accuracy and sample representativeness when compared to the two methods. This approach yields promising outcomes for field sampling in digital soil mapping. By simultaneously optimizing both sample representativeness and cost constraints, it holds potential as a valuable decision support tool for soil surveyors facing sampling designs in poorly accessible areas. Graphical abstract: https://ars.els-cdn.com/content/image/1-s2.0-S0016706124001174-ga1.jpg