Essential Geography Cheat Sheet

Getting Your Data Right Before You Map It

Most people open a GIS program, throw in three shapefiles, and start making maps without realizing the files are speaking different languages. This is where an Essential Geography Cheat Sheet actually earns its name. Not as something decorative you pin above your desk, but as a practical reference for the moments when your coordinates look wrong and you cannot immediately tell whether it is a projection issue, a datum problem, or a bad export from the source system. I keep a condensed version open in a browser tab while I work. The full version is longer, but I am not here to sell you anything. The link to the downloadable PDF is at the bottom. Let me explain why most people skip reading it until they already have a broken map on their screen, and what you should be checking before that happens.

Essential Geography Cheat Sheet

The first thing you need to understand is that coordinate reference systems are not interchangeable labels. They are mathematical transformations. When you reproject a layer from WGS 84 (EPSG:4326) into Web Mercator (EPSG:3857), you are changing both the angular coordinates and the underlying ellipsoid assumptions, and area will distort. A shapefile that looks correct in QGIS at first glance can hide serious scale errors if you measure distances in the projected layer without converting it to an equal-area CRS. This is not theoretical. I lost half a day once because a client sent me parcel data in NAD 27, UTM Zone 15 North, and I calculated drainage basin areas directly in that projection without realizing the datum was pre-North American Datum rather than the modern NAD 83 version. The positional shift between those datums in that region alone accounts for roughly two meters of horizontal error, which cascaded into incorrect elevation model inputs downstream. That mistake taught me the single most useful habit: always verify the source CRS, check the datum explicitly, and then decide whether reprojecting is even necessary. You do not need to project every layer into the same CRS just to display them together on a basemap. Modern desktop software will on-the-fly reproject, but on-the-fly reprojection introduces visual inaccuracies and slows rendering. Export your active analysis layer to the target CRS before running geoprocessing tools. The difference between working in the native CRS and a forced CRS is usually a few extra minutes of cleanup, and it prevents silent topology errors that surface hours later.

What to Put on the Sheet

An effective reference needs to cover four categories. The first is projections and when to use each one. Web Mercator is acceptable for web visualization and tile hosting. It is unacceptable for area measurement, distance calculation, or any spatial analysis that depends on scale fidelity. For North America, Alaska Albers Equal Area Conic is the standard for statistical mapping. For Europe, Lambert Conformal Conic is common. For global datasets, Equirectangular or the Robinson projection may be appropriate if you only need a visual summary, but do not claim those are accurate representations of anything. The second category is datums. WGS 84 and GDA 2020 are compatible to within centimeters for most practical purposes. NAD 83 and WGS 84 diverge slightly over time due to tectonic plate motion, and using an outdated transformation grid can shift coordinates by tens of centimeters in some regions. If your data spans multiple datums, do not guess the transformation. Use the grid-shift files provided by your national survey authority, or leave the data in its original datum until the final deliverable requires a specific output. The third category is units and their relationship to CRS. Geographic CRS store angles in degrees. Projected CRS store linear units, typically meters, but sometimes feet or survey chains. A common error is computing a buffer in meters around a layer stored in degrees, which produces buffers measured in decimal degrees instead. The fix is straightforward: reproject the layer first, then apply the buffer in the projected CRS. Always check the units field in your software before running any distance-based tool.

The fourth category is common EPSG codes and their regional coverage. The cheat sheet lists roughly 60 of the most frequently encountered codes. You do not need to memorize all of them. What you need to memorize is the pattern. EPSG codes in the 4200 range are geographic. Codes in the 32600 to 32700 range are UTM zones. Codes in the 2100 range are often State Plane zones in the United States. When you see an EPSG code you do not recognize, look it up in the EPSG Geodetic Parameter Dataset rather than assuming it is correct based on its number alone.

A Practical Workflow

Start every project by opening the data properties and recording the CRS, datum, and units. If a file has no CRS definition, treat it as suspect and contact the provider before proceeding. A missing CRS is worse than a wrong one because a wrong CRS raises an alarm immediately. A missing CRS lets you build an entire analysis on unverified assumptions. Once you confirm the source CRS, define the output CRS for your final deliverable. Reproject once, not repeatedly. Each reprojection introduces rounding error and can degrade precision, especially when cycling between geographic and projected systems. Run your geoprocessing in the output CRS. If you must keep intermediate results in a different CRS, export them explicitly between stages rather than relying on automatic transformations. For raster data, the same rules apply. Elevation models stored in geographic CRS cannot be used for slope or aspect calculations without reprojection, because the horizontal and vertical units must be comparable. Verify pixel size after reprojection as well. Resampling algorithms matter here. Nearest neighbor preserves categorical values. Bilinear interpolation is appropriate for continuous surfaces like temperature or elevation. Cubic convolution can introduce ringing artifacts near sharp edges, so avoid it for DEM-derived products unless you validate the output against the original.

Where This Approach Fails

An Essential Geography Cheat Sheet does not solve every problem. It will not help you when the source data itself is corrupted, when feature coordinates were digitized by hand with inconsistent projections, or when a dataset uses a custom local CRS that has no standard EPSG assignment. In those cases, the cheat sheet is still useful for identifying what you are missing, but the resolution requires contacting the data provider, examining the raw coordinates, and sometimes rebuilding the CRS definition manually from metadata files or .prj text entries. It also fails silently in cases where a transformation is applied automatically without your knowledge. Some software platforms default to WGS 84 for any unrecognized input and convert everything behind the scenes. This is dangerous because it appears to work until you discover that your coordinates no longer match the source document. Disable auto-reprojection in your project settings unless you have a specific reason to keep it enabled. If you are working with high-precision survey data, GNSS observations, or legal boundary descriptions, a general cheat sheet is insufficient. You need the transformation parameters specific to your region, and you should consult the official geodetic authority for your country. The cheat sheet is a starting point, not a replacement for jurisdiction-specific documentation.

Downloading the Reference

The full document includes tables for projection selection by continent, a checklist for validating CRS before analysis, and examples of common mistakes with their corrections. It is written for practitioners who already know the basics and want to avoid the errors that take hours to debug. Download Essential Geography Cheat Sheet (PDF) Keep it open. Use it before you start, not after your map looks wrong.