How do you handle multiple map projections in a single GIS map or analysis?

1 Understand your data

The first step is to understand the map projection of each data layer or source that you use in your GIS project. You can usually find this information in the metadata, the documentation, or the attribute table of the data. You should also check the geographic coordinate system (GCS), which is the reference system that defines the latitude and longitude of the data. The GCS and the map projection together define the spatial reference of the data, which is essential for aligning and overlaying different data layers.

2 Choose a suitable projection

The next step is to choose a suitable map projection for your map or analysis, based on the spatial extent, the thematic focus, and the intended audience of your project. For example, if you are mapping a large area that spans multiple continents or hemispheres, you may want to use a global projection that preserves area or shape, such as the Robinson or the Winkel Tripel projection. If you are mapping a smaller area that covers a single country or region, you may want to use a local projection that preserves distance or direction, such as the UTM or the Lambert conformal conic projection. If you are mapping a specific phenomenon or theme, such as climate change, population density, or land use, you may want to use a projection that highlights or minimizes certain aspects of the data, such as the Gall-Peters or the Mercator projection.

3 Reproject or transform your data

The third step is to reproject or transform your data layers to match the map projection that you have chosen for your map or analysis. Reprojection is the process of changing the map projection of a data layer, while transformation is the process of changing the geographic coordinate system of a data layer. You can use various tools and functions in your GIS software to perform these operations, such as the Project or the Define Projection tools in ArcGIS, or the Reproject Layer or the Set Layer CRS tools in QGIS. You should always make a backup copy of your original data before reprojecting or transforming it, and check the quality and accuracy of the output data.

4 Use on-the-fly projection

The fourth step is to use on-the-fly projection, which is a feature that allows your GIS software to display different data layers with different map projections in a single map view, without permanently changing their spatial reference. This can be useful for quickly comparing or visualizing different data layers, or for testing different map projections without reprojecting or transforming your data. However, you should be aware that on-the-fly projection may introduce some errors or distortions in your data, especially if the data layers have very different spatial references or extents. Therefore, you should always use on-the-fly projection with caution, and verify your results with other methods.

5 Document your projection

The final step is to document your map projection and the steps that you have taken to handle multiple map projections in your map or analysis. You should include this information in your metadata, your report, your presentation, or your publication, depending on the format and purpose of your project. You should also include a map legend, a scale bar, a north arrow, and a citation of the data sources and the map projection that you have used. This will help you and your audience to understand the spatial context and the limitations of your map or analysis, and to avoid any confusion or misinterpretation of your data and results.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?