\BeginKnitrBlock{rmdwarn}<div class="rmdwarn">Do not attempt to run geoprocessing queries or any other complex operations in postgis on large data tables such as the original priority habitat layers until you are more experienced. A key advantage of postgis is that it is possible to run analysis on very large data sets. In theory all the priority habitat patches further than fifty meters from all the paths in Dorset could be extracted from the data with one query. In practice this query could take hours to run through, so techniques involving prototyping are needed for big data operations. See section x on big data operations in postgis (which I have not written yet!)</div>\EndKnitrBlock{rmdwarn}