Projecting NLCD data as WGS84 using QGIS?

Getting NLCD Data into WGS84 with QGIS: A Human’s Guide

So, you’ve got some National Land Cover Database (NLCD) data, and you need to wrangle it into WGS84, the go-to coordinate system for, well, pretty much everything these days. No sweat! NLCD is awesome – a fantastic, consistent land cover dataset spanning the whole US at a decent 30-meter resolution. But it often lives in its own little world, projection-wise. This guide will walk you through the surprisingly simple process of getting it playing nicely with WGS84 using QGIS, that amazing (and free!) open-source GIS software.

The Projection Predicament

Here’s the deal: NLCD data typically comes in Albers Conical Equal Area projection, specifically NAD83 / Conus Albers (EPSG:5070). It’s great for minimizing distortion across the continental US, but not so great when you’re trying to overlay it with, say, GPS data or a web map that’s using good old latitude and longitude in WGS84. That’s why we need to reproject. Think of it like translating between languages – you need a common reference point.

Reprojection: Step-by-Step (No Jargon!)

Alright, let’s get our hands dirty. Here’s how to make the magic happen in QGIS:

The “Save As” Shortcut

There’s another way to skin this cat:

Pro Tips and Gotchas

• Nearest Neighbor is Your Friend: Seriously, for NLCD, use “Nearest Neighbor” resampling. I can’t stress this enough.
• Big Data = Big Processing Time: NLCD files can be hefty. If you’re just working with a small area, clip the raster before reprojecting. It’ll save you a lot of time.
• Datum Transformations: The Hidden Culprit: If things look wonky after reprojecting (slanted, misaligned), the datum transformation might be off. Dig into the settings and make sure it’s correct. This is a more advanced topic, but worth knowing about.
• On-the-Fly: Use with Caution: QGIS can reproject layers “on-the-fly,” but it’s generally better to actually reproject the data. It’s more accurate and avoids potential performance issues.

Troubleshooting Common Issues

• Vanishing Layer: If the layer disappears after setting the CRS, zoom to the layer’s extent. QGIS sometimes gets confused.
• Data Looks Skewed: Double-check that you’ve got the original CRS right. That’s the most common cause of misalignment.

Wrapping Up

Reprojecting NLCD data to WGS84 in QGIS is a skill every GIS user should have. It’s not rocket science, but paying attention to the details – especially resampling and datum transformations – is key to getting accurate results. Now go forth and map!