Choosing the right mapping scale for stream assessment

Posted on March 26, 2010

Aquatic conservation biologists typically rely on maps of streams and lakes in a geographic information system.  Environmental data are produced by digitising printed maps, but the fact that these are available at different scales means that the user must choose the most appropriate dataset for a particular purpose.  The effects of mapping scale on in-stream and streamside assessments have received surprisingly little attention.  Oregonresearchers compared stream habitat data from maps at two spatial scales that are both widely used in aquatic management.  Data collected from 1 : 100,000 maps (where 1 cm on the represents 1 km on the ground) gave significantly higher estimates of  channel depth, channel width, sinuousity, stream discharge and the numbers of deep and complex pools than those extracted from maps at a higher resolution (1 : 24,000, where 1 cm on the map = 0.24 km).  In contrast, estimates of channel length and gradient were greater when based on the higher resolution data.  Although many fish, amphibian and invertebrate species depend heavily on small headwater streams, such channels are under-represented on low resolution maps: for example, 1: 100,000 datasets omit up to 80% of the total stream length in a given area.  These findings emphasise the importance of quoting the spatial scale of source data in species status reports and other conservation publications.

 Reference: Vance-Borland, K., Burnett, K., & Clarke, S.  2009.  Influence of mapping resolution on assessments of stream and streamside conditions: lessons from coastal Oregon, USA.  Aquatic Conservation: Marine and Freshwater Ecosystems 19, 252–263.