The transition to climate-neutral energy systems by 2050, as mandated by the Energy Efficiency Directive, requires a profound transformation in how heating and cooling are produced and delivered. District heating and cooling networks (DHCNs) play a strategic role in this transition by enabling efficient sector coupling, supporting the integration of renewable and waste heat (WH) sources, and offering thermal flexibility and storage capacity. These features make DHCNs particularly effective in decarbonizing urban areas, enhancing overall energy efficiency, and increasing the share of renewables in the heating and cooling sector. This work presents a geospatial methodology for mapping waste heat potential from both industrial and tertiary sectors, providing critical input for the modernization and expansion of existing DHCNs. The approach is based on a structured GIS framework that integrates open data, statistical analysis, and literature-based datasets to identify and quantify WH sources. Industrial WH mapping combines facility-level data, such as geographic location, NACE code, and number of employees, with the Danish WH dataset and a novel interpolation routine to estimate annual WH potential across three temperature levels. Urban WH mapping targets tertiary buildings, such as supermarkets, hospitals, swimming pools, and ice rinks, using GIS geometries and descriptive metadata. The estimation is based on gross floor area and category-specific coefficients derived from real-world energy use data and technical benchmarks. The outcome is a harmonized spatial inventory of WH sources with hourly-resolution profiles, offering a robust foundation for energy system modelling, network design, and decarbonization scenario planning.