Point of view: error estimation in field assignment of land-cover types

Abstract

Questions: Substantial variation between observers has been found when comparing parallel land-cover maps, but how can we know which map is better? What magnitude of error and inter-observer variation is expected when assigning land-cover types and is this affected by the hierarchical level of the type system, observer characteristics, and ecosystem properties? Study area: Hvaler, south-east Norway. Methods: Eleven observers assigned mapping units to 120 stratified random points. At each observation point, the observers first assigned a mapping unit to the point independently. The group then decided on a 'true' reference mapping unit for that point. The reference was used to estimate total error. 'Ecological distance' to the reference was calculated to grade the errors. Results: Individual observers frequently assigned different mapping units to the same point. Deviating assignments were often ecologically close to the reference. Total error, as percentage of assignments that deviated from the reference, was 35.0% and 16.4% for low and high hierarchical levels of the land-covertype system, respectively. The corresponding figures for inter-observer variation were 42.8% and 19.4%, respectively. Observer bias was found. Particularly high error rates were found for land-cover types characterised by human disturbance. Conclusions: Access to a 'true' mapping unit for each observation point enabled estimation of error in addition to the inter-observer variation typically estimated by the standard pairwise comparisons method for maps and observers. Three major sources of error in the assignment of land-cover types were observed: dependence on system complexity represented by the hierarchical level of the land-cover-type system, dependence on the experience and personal characteristics of the observers, and dependence on properties of the mapped ecosystem. The results support the necessity of focusing on quality in land-cover mapping, among commissioners, practitioners and other end users. Taxonomic reference: Lid & Lid (2005) for vascular plants. Syntaxonomic reference: Halvorsen et al. (2015) for land-cover types. Abbreviations: ED = Ecological distance; GLM = Generalised linear model; LCE = Local complex environmental variable; NiN = Nature in Norway; TPI = Topographic position index.

Bridging theory and implementation – Testing an abstract classification system for practical mapping by field survey and 3D aerial photographic interpretation

“…The bedrock consists of acidic granite. The landscape is dominated by rounded hills with thin soil coverage, broken up by abrupt fissure valleys formed in weaker lines of the bedrock (Eriksen et al 2019). The valleys are covered with fine-grained marine sediments containing remains of calcareous shells (NGU n.d.).…”

“…This gives a characteristic landscape with large areas of bare rock and sparse trees, interrupted by narrow valleys with productive forests. The vegetation in the study area is structured partly by natural processes and partly by cultural influence (Eriksen et al 2019). Scattered and shallow soils, exposure to seawater and strong winds have generated large open areas.…”

“…Bare rock is often found in a mosaic with a thin soil layer dominated by heather (Calluna vulgaris). In convex land forms with slightly deeper soils, though still exposed to drought, Scots pine (Pinus sylvestris) dominates the sparse tree layer (Eriksen et al 2019). Due to poor drainage, small patches of wetland dominated by bog myrtle (Myrica gale) occur in local depressions in the otherwise convex bedrock.…”

“…Due to poor drainage, small patches of wetland dominated by bog myrtle (Myrica gale) occur in local depressions in the otherwise convex bedrock. Most of fertile land was utilized for agriculture during the 1850-1930 population peak but most agricultural areas were later abandoned, allowing spontaneous regrowth of forest (Eriksen et al 2019).…”

“…The land cover classification implemented in the study followed NiN version 2.0 (Halvorsen et al 2015;Ullerud et al 2018;Eriksen et al 2019). The NiN system divides terrestrial Norway into 59 major ecosystem types and 444 basic ecosystem types.…”