Question: Modified camera with 25A red filter - NDVI needs calibration- Any Ideas?

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Tbtouaki asked on September 24, 2014 09:56
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What I want to do

I am trying to measure NDVI with balloon mapping on several burned sites (different time periods after the wildfire).

My attempt and results

I have a two camera set: One with true color

CORTE_A076_2014_07_18_TC.jpg

The second modified, I removed the IR block filter and inserted a red 25A filter, for IR images.

CORTE_A076_2014_07_18_IR.jpg

I put them both on the balloon simultaneously, and shoot burned sites on different recovery stages.

Questions and next steps

I want to measure NDVI on those pictures, but my values are low, even when the trees survived the fire.

I've read here that a camera modified like mine gives us pure NIR on band 1, NIR+RED on band 2, and RED on band 3. Because it is easier to use bands from just one picture from the same camera, I've measured NDVI using only the NIR camera's picture (B1-B3/B1+B3).

NDVI_CORTE_A076_2014_07_18_IR.jpg

I've checked if I might have made a mistake with the picture's bands but I haven't because when I invert the bands I get lower NDVI values for the living plants and higher values for the soil and dead plants. NDVI_CORTE_A076_2014_07_18_IR_invertido.jpg

Should I use the RED band from the true color camera? It is hard to over-lapse all my pictures. But the values are still very negative when compared with the expected. In fact the values are really similar when not equal to the NDVI measured using only the modified camera.

NDVI_CORTE_A076_2014_07_18_IR_TC.jpg

I need to calibrate the photos or the NDVI. Any ideas? Could really use some help



balloon-mapping near-infrared-camera ndvi infrared infragram super-red

question:infragram question:white-balance



5 Comments

cfastie over 3 years ago

I have not previously seen NDVI images quite like these, so the first thing to check is that the expression you include above:

I've measured NDVI using only the NIR camera's picture (B1-B3/B1+B3)

is being entered as ((B1-B3)/(B1+B3)).

The second issue is that the photos from the Wratten 25A camera will not easily produce meaningful NDVI unless a custom white balance has been done on the camera. Not all cameras allow this.

The third issue is that NDVI results will be more useful if different cameras are used to capture NIR and red. Since you are flying two cameras, results could be better, but you have to synchronize the shutters and point the cameras at the same scene.

Chris

liz over 3 years ago

Hey @Tbtouaki , how did this turn out?

Tbtouaki almost 3 years ago

Hi I had wrong white balance for my camera. Thanks everyone for the help. Sorry for the trouble If anyone has any idea on how to save pictures taken with wrong white balance I have quite a few I'd like to save. However from now on, of course, I'll use correct white balance. Thanks Best regards, Rita

Jakub over 2 years ago

Hi!

@Tbtouaki Could You explain, how You calibrated the WB on Your camera?

Best regards, Jakub

Tbtouaki over 2 years ago

Hi @Jakub

I re-read lots and lots of posts here, but basically depending on the IR filter you use, you should use a color card, a red card to set your white balance right. It's more or less to tell your camera what color is the baseline...

Check theses posts: http://publiclab.org/wiki/infrablue-white-balance http://publiclab.org/notes/cfastie/07-17-2013/balancing-act

What I did later, was figuring out a way to save photos taken with the wrong settings. I am still trying to publish it. But you should always have the right white balance before taking any photos.

Hope this helps. Best regards, Rita

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