Calculating NDVI in QGIS

Apr 30, 2024 | Forestry, NAIP, QGIS

Titling these articles is getting more difficult. I almost called this my “Getting my Junior Forestry Badge” because that’s what I told one client the other day as I confidently misidentified another tree on a walk. A forestry person I am not – but I’ve gotten pretty decent with INaturalist.

For all my weird QGIS tricks – I can’t remember the last time I looked at NDVI. What is NDVI? It’s the Normalized Difference Vegetation Index. How often do I use it? Not often. Actually – hardly in the last bit as all my info comes from field work but in a push to “work smarter” I’m attempting to drag one client along for the ride.

So I grabbed some 2022 4 band NAIP and played around with QGIS and the Field Calculator. With NAIP you take Band 4 and subtract Band 1 and divide that from Band 4 added to Band 1. There isn’t a button in QGIS so you have to open field calculator and get to clicking.

These photos were taken in Feb of 2022. In February all the hardwoods are still dormant. So you have the NAIP:

NDVI via Raster Calculator:

Why not pull some LIDAR in and look at height:

Why does it matter? We received a stand map that was probably a good 20 years out of date. I had to pull that information out of a GeoPDF. When you look at the stands below (there was very little data beyond geometry), everything is shaded in and when looking at the the other imagery – everything has been cut/thinned with the exception of the streams bank.

This isn’t going to save field work in this case. We have 2018 LIDAR and 2022 Imagery and no record of forestry work in the last few years. I could pull a whole set of NAIP and check it year by year but I’m going to wait until the first field check and then maybe we do some research to see what happened when. Satellite imagery is a possibility but unless it’s $3 dollars the client won’t go for it.

What was the point? Honestly, more curiosity than anything. I have all this readily available data and can I make it all useful? Can I squeeze more information out of this that I hadn’t cared about before. I think I can. How much can I squeeze out? Stay Tuned.

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