The Coordinate Reference System (CRS) tells the software reading the spatial data where the data are located on the globe. For an introduction to this topic, see this [article](https://www.earthdatascience.org/courses/earth-analytics/spatial-data-r/intro-to-coordinate-reference-systems/). For most web applications use WGS 84 (EPSG code 4326), a geographic coordinate system. For any spatial calculations, you will instead want a projected coordinate system. See [here](https://www.earthdatascience.org/courses/use-data-open-source-python/intro-vector-data-python/spatial-data-vector-shapefiles/geographic-vs-projected-coordinate-reference-systems-python/) for more on the difference between geographic and projected coordinate systems. The Universal Transverse Mercator (UTM) CRS is a common a projected coordinate system. It's units are meters, so any area calculations are in square meters, distance is in meters, etc. To find the UTM zone of your geography, use this [application](https://mangomap.com/robertyoung/maps/69585/what-utm-zone-am-i-in-#). When a geography spans more than one zone, this CRS will still work but it will be skewed as you get further outside the selected zone. There are also a number of projected coordinate systems devised for different geographies, so you can generally find one that is tailored to the bounds of your geography. To find the EPSG code of the UTM Zone, search [this database](https://spatialreference.org/). Alternatively, use World Equidistant Cylindrical (EPSG code 4087) for imprecise calculations without worrying about which UTM zone to use. When an EPSG code is not available, you must use the proj4 string to set the crs when reading or immediately thereafter. Then you can convert to WGS84. For example, Guatemala has an official projection that is not regeistered with an EPSG code. The proj4 string is ``` "+proj=tmerc +lat_0=0 +lon_0=-90.5 +k=0.9998 +x_0=500000 +y_0=0 +ellps=WGS84 +units=m +no_defs" ``` To set the projection in Python use ```python gdf.crs = "+proj=tmerc +lat_0=0 +lon_0=-90.5 +k=0.9998 +x_0=500000 +y_0=0 +ellps=WGS84 +units=m +no_defs" ``` where `gdf` is a [[geopandas]] [[geodataframe]].