2. Represent the range of environmental conditions for each habitat
Coastal areas will often possess gradients of water qual- ity and suspended sediment concentration. Changes in these parameters across an image can lead to spectral con- fusion during image classification and misassignment of habitat categories. To mitigate this effect, surveys should represent a wide cross-section of each physical environ- ment. This will provide further field data to train the image classification and provide data for accuracy assessment (to highlight the extent of the inaccuracies where they occur). As an example, an unsupervised classification of Landsat TM imagery of the Caicos Bank identified a spe- cific habitat type on both sides of the Bank (some 40 km apart). Surveys near the field base identified this habitat as seagrass and it would have been easy to assume that all similar-looking habitats in the imagery fell into this class.
58 Remote Sensing Handbook for Tropical Coastal Management
However, field surveys at the opposite side of the Bank identified a very different habitat type (organic deposits on sand), thus reinforcing the need for extensive field work.
3. Choose a sampling strategy
To ensure that all habitats are adequately represented by field data, a stratified random sampling strategy should be adopted (Congalton 1991). The unsupervised image clas- sification and map of main physical environments can be used to stratify sampling effort. A similar number of sites should be obtained in each area. Truly random sampling within each stratum (area) is likely to be prohibitively time-consuming because the boat would have to visit ran- domly selected pairs of coordinates, thus incurring waste- ful navigation time. In practice, driving the boat throughout each area with periodic stops for sampling is likely to be adequate. The main limitation to any field sur- vey is cost/time. While every attempt is made to obtain the maximum amount of data, Congalton (1991) recom- mends that at least 50 sites of each habitat be surveyed for accuracy assessment purposes. We feel that an additional 30 sites should be visited for use in image classification (Chapter 19).
4. Estimate costs of field survey
Field surveys are expensive and not all of the costs incurred in gathering field data and relating it to remotely sensed data are immediately obvious. However, a full analysis of field costs is vital when designing a remote sensing campaign to ensure that realistic budgets and work schedules are planned. A generalised discussion of costs is presented here. Detailed advice on planning a remote sensing field campaign in terms of cost and the actual costs incurred in mapping the habitats of the Turks and Caicos Islands are given in Chapter 19.