How to add Lat/long columns to a point layer which is not in WGS84?

Decoding Lat/Long: Adding Coordinates to Your Point Data (Even When It’s Messy)

Ever wrestled with geospatial data that just refuses to play nice? You’re not alone. One of the most common headaches is dealing with point layers that aren’t in the standard WGS84 coordinate system—the one your GPS uses. It’s like trying to fit a square peg in a round hole if you try to directly pull latitude and longitude values from these layers. But don’t worry, it’s totally solvable.

The trick is that your data speaks a different spatial language. Instead of WGS84, it might be in something like a UTM zone, or a State Plane system – all perfectly valid, just… different. So, before you can start plotting those points on a map using standard lat/long, you need to translate. Think of it like converting kilometers to miles.

Here’s how to wrangle those coordinates into shape, using tools you probably already have in your GIS software (like QGIS or ArcGIS Pro). The exact buttons might be in slightly different places depending on your software, but the core idea is the same.

First things first: Know Thy Data! You absolutely must figure out what coordinate system your point layer is currently using. This isn’t a guessing game. Check the metadata – that’s the information about your data, usually in a separate file or accessible through your GIS software. Look for a .prj file lurking alongside your data. If it’s missing, you might have to do some detective work, consulting whoever provided the data or researching the region it covers. Trust me, getting this wrong will send you down a rabbit hole of errors. I learned this the hard way once when I assumed a dataset was in a common CRS, only to realize later it was a custom local grid. Hours wasted!

Okay, got the CRS? Great! Now, Reproject It. This is where the magic happens. Find the “Reproject” or “Transform” tool in your GIS software. Tell it your input layer (the point layer you’re working with), the current CRS you just identified, and the target CRS: WGS84 (EPSG:4326). This process mathematically transforms the coordinates, shifting each point from its original spatial framework to the equivalent spot on a WGS84 globe. Crucially, create a new layer as the output! Never overwrite your original data; you might need it later.

Next, Add Those Columns. Now that your data “speaks” WGS84, you can finally add those latitude and longitude columns. Open your reprojected point layer’s attribute table (that’s where the data about each point is stored). Create two new fields: “latitude” (or “lat,” whatever you prefer) and “longitude” (“long”). Make sure these fields are numeric, so you can store decimal values. “Double” or “Float” are good choices.

Time for the fun part: Calculate! This is where you tell the software to fill those new columns with the actual coordinate values. Use the “Field Calculator” (or whatever your software calls it). You’ll use a little expression to grab the X and Y coordinates from each point:

• Longitude: Use $x or x( $geometry ). This pulls the X-coordinate, which is the longitude in WGS84.
• Latitude: Use $y or y( $geometry ). This grabs the Y-coordinate, which is the latitude in WGS84.

Apply these calculations to the correct columns, and boom! You should have latitude and longitude values for every point.

Double-Check Your Work! Don’t just blindly trust the software. Take a look at the attribute table and the points on the map. Do the values seem reasonable for the area? Pick a few known locations and compare their coordinates to what you’ve calculated. This sanity check can save you from embarrassing mistakes later on.

A Few Things to Keep in Mind:

• Datum Shenanigans: Sometimes, reprojection involves something called a datum transformation. Datums are like slightly different versions of the Earth’s shape. Your GIS software usually handles this automatically, but it’s good to be aware of, especially if you’re working with super-precise data.
• Accuracy Trade-offs: Reprojecting can introduce tiny distortions, depending on the projections involved. For most applications, it’s not a big deal, but if you’re doing something where millimeters matter, be aware of it.
• Big Data Blues: Reprojecting huge datasets can take time. Use efficient data formats (like GeoPackage) and tweak your software settings to speed things up.

Help! It’s Not Working!

• Wrong CRS: This is the #1 culprit. Double, triple-check that you’ve identified the correct current CRS.
• Values Swapped: Did your latitude and longitude get mixed up? Double-check those field calculator expressions.
• Data in the Wrong Place: If your reprojected data ends up halfway across the world, it’s likely a datum issue. Make sure your GIS software is handling datum transformations correctly.

Adding lat/long columns to your point data might seem intimidating at first, but once you get the hang of it, it’s a straightforward process. And it unlocks a world of possibilities for analyzing and visualizing your geospatial data. So go forth, reproject with confidence, and happy mapping!