Citation:

Wade, T G., J D. Wickham, M S. Nash, A C. Neale, K. H. Riitters, AND K B. Jones. A COMPARISON OF VECTOR AND RASTER GIS METHODS FOR CALCULATING LANDSCAPE METRICS USED IN ENVIRONMENTAL ASSESSMENTS. PHOTOGRAMMETRIC ENGINEERING AND REMOTE SENSING 69(12):1399-1405, (2003).

Impact/Purpose:

The primary objectives of this research are to:

Develop methodologies so that landscape indicator values generated from different sensors on different dates (but in the same areas) are comparable; differences in metric values result from landscape changes and not differences in the sensors;

Quantify relationships between landscape metrics generated from wall-to-wall spatial data and (1) specific parameters related to water resource conditions in different environmental settings across the US, including but not limited to nutrients, sediment, and benthic communities, and (2) multi-species habitat suitability;

Develop and validate multivariate models based on quantification studies;

Develop GIS/model assessment protocols and tools to characterize risk of nutrient and sediment TMDL exceedence;

Complete an initial draft (potentially web based) of a national landscape condition assessment.

This research directly supports long-term goals established in ORDs multiyear plans related to GPRA Goal 2 (Water) and GPRA Goal 4 (Healthy Communities and Ecosystems), although funding for this task comes from Goal 4. Relative to the GRPA Goal 2 multiyear plan, this research is intended to "provide tools to assess and diagnose impairment in aquatic systems and the sources of associated stressors." Relative to the Goal 4 Multiyear Plan this research is intended to (1) provide states and tribes with an ability to assess the condition of waterbodies in a scientifically defensible and representative way, while allowing for aggregation and assessment of trends at multiple scales, (2) assist Federal, State and Local managers in diagnosing the probable cause and forecasting future conditions in a scientifically defensible manner to protect and restore ecosystems, and (3) provide Federal, State and Local managers with a scientifically defensible way to assess current and future ecological conditions, and probable causes of impairments, and a way to evaluate alternative future management scenarios.

Description:

GIS-based measurements that combine native raster and native vector data are commonly used to assess environmental quality. Most of these measurements can be calculated using either raster or vector data formats and processing methods. Raster processes are more commonly used because they can be significantly faster computationally than vector, but error is introduced in converting vector data to raster. This conversion error has been widely studied, but the impact on environmental decision making has not been addressed. We examined four GIS-based measurements of environmental quality in approximately 1000 watersheds in the state of Maryland and Washington, D.C. Each metric was calculated using vector and raster methods and estimated values were compared using a paired t-test, Spearman rank correlation and cluster analysis. Paired t-test results showed significant differences between methods for three of the four metrics. Spearman ranks were nearly identical between methods for all four metrics. Cluster analysis also reported nearly identical results between methods, with over 98% of watersheds assigned to the same group for three of the four methods. The paired t-test results are consistent with other studies that show credible differences in metrics that depend on whether estimation was done in a raster or vector format, but the Spearman rank correlations and the cluster analysis suggest that these quantitative differences would not influence environmental decisions.

Record Details:

Document IDs: