# The Impact of Spatial Resolutions on Nature-Based Solution Suitability Mapping for Europe
## 1 Introduction  
Nature-Based Solutions (NBS) have been gaining a significant role in disaster risk reduction, climate change mitigation and adaptation, as well as in sustainable development [1]. Their rising importance in policies and practice in Europe can be ascribed to their character as natural measures which simultaneously offer co-benefits for the socio-ecological system [2, 3]. The increasing recognition of NBS in Europe is linked to the ambition to implement them at various scales in rural and urban areas [4]. To enhance the uptake of NBS, it is necessary to support the decision-making of policy makers, practitioners, associations, and further actors who are not experts in this field but are engaged in the planning and implementation of NBS because they have specific requirements (e.g., for soil) that need to be considered. Hence, to determine the suitability of a particular NBS for an area of interest, several criteria need to be fulfilled. For decision-support, a preliminary assessment can be conducted based on local characteristics (e.g., soil, land cover, vegetation, and built-up areas) to gain an initial understanding of the suitability of an NBS. This type of multi-criteria pre-assessment in NBS is referred to as suitability mapping [5], (site) suitability analysis/assessment [6], or, alternatively, as opportunity mapping [7]. When conducting acomplete suitability analysis for real-world application, several additional criteria which are not covered in the pre-assessment may also need to be considered. These additional criteria may be social aspects (e.g., acceptance of the NBS by the public), legal factors (e.g., land ownership, policies in context of the permission of NBS), and the economic component (e.g., funding for the land reimbursement, implementation, and maintenance) [5; 8; 9; 10; 11]. Existing suitability mapping tools and studies vary in their scale, although the following core approach remains similar: focus on urban areas and city scale [11], perform subnational scale analysis primarily for rural areas [12], map NBS suitability at a larger scale (e.g., national) [5], and at a global scale [7]. However, NBS suitability mapping has not been approached at the European level yet. In response to this gap and the increasing promotion of NBS in Europe, as well as the complexity of decision-making for NBS, we developed the NBS Toolkit [13] (within the H2020 OPERANDUM project) to support decision-making for NBS in rural and urban areas. The NBS Toolkit aims to recommend NBS locations for the entire European region at the local scale. The suitability mapping of the toolkit builds on open-source data at the European scale, but the available datasets can vary greatly in resolution. For instance, datasets for land use and land cover (LULC) are available in 100 m, 50 m, and down to 2 m resolution. In order to gain evidence on the appropriateness of these different open-source datasets for the suitability mapping at the regional scale, this study aims to assess the influence of different spatial resolutions on the resulting suitability maps. Differences in spatial resolution can be further related to differences in accuracy and data size, and have been found to impact the computation outcome, i.e., in the context of hydrological modelling [14; 15; 16; 17; 18], ecosystem service assessments [19; 20], or agricultural suitability mapping [21]. These studies discussed the result accuracy, the adequateness of fine resolution in the context of the desired goal of a study, and the feasibility of fine resolution, considering computation capacity. In terms of accuracy of the results, it was found that accuracy increases equally with finer spatial resolution [19], provided that all input layers are of a similar resolution [15]. Considering the desired output scale, fine spatial resolution could be a precondition, i.e., for urban suitability mapping due to greater land use heterogeneity [19]. Hence, which output scale is desired should be considered [18].