Thematic base layers
Thematic base maps primarily serve as input to further GIS analyses. However, due to their intuitive nature, they are also frequently used as a graphical information carrier to facilitate the communication between stakeholders and decision makers. In that capacity they are used to provide an overview of all relevant project features and foster a common understanding of location, scale and information content of a project region.
A thematic base map of the basin or sub-basin in question accurately depicts the main thematic features of interest in that region, including but not limited to hydrological networks, administrative boundaries, all relevant infrastructure (e.g. roads, bridges, dams, channels, etc.), settlements, basin delineations and other features. It would often also illustrate elevation through contour lines or coloration. It serves as a basic information carrier for activity planning and further GIS analysis. Thematic maps are produced from existing national and/or international and/or global cartographic information products and user‐supplied geodata. As such its accuracy and topicality depends on that of the input data. In cases where such data is scarce or of insufficient reliability, the objects of interest may be derived from high resolution satellite imagery either through classification or digitization, depending on the project scale and available resources. In any case, a recently acquired high resolution satellite images should complement the topographic base map and serve as a reference for its validation.
Thematic base maps are designed to present their information in an intuitive manner and thus facilitate project related communication between stakeholders of various backgrounds.
The quality of a thematic base map strongly depends on the availability of accurate and topical cartographic and geospatial data containing the features of interest. Such information may not be available for a specific project location and need to be generated from optical satellite imagery which in turn must be available in sufficiently high resolution. The latter requirement is generally well covered through available Sentinel 2 and Landsat archives which are suitable for basin and sub-basin scales.
The increasing availability of high resolution optical satellite imagery and, in some cases, GIS ready thematic base data for project locations eases the production of reliable thematic base maps. The high revisit frequency of, of example, the Sentinel satellites of currently about 5-10 days significantly increases the chances for topical, cloud free coverages needed for map validation.