Public Lab Research note


NVDI with canon a2500

by gpenzo | February 09, 2014 21:45 | 394 views | 5 comments | #10024 | 394 views | 5 comments | #10024 09 Feb 21:45

Read more: publiclab.org/n/10024


What I want to do

My goal is to make nvdi images of farmland. Hopefully the quality will be good enough to help farmers locate plant stress in the field. I think the best quality NDVI images will be made with two cameras. I'm using two canon a2500 cameras. The reason for a2500: the cheapiest canon camera i could buy.

My attempt and results

Usual steps for the camera remove: IR filter. It is difficult to get a watter 85A filter arround here. So I use a massa 850 nm IR filter.

Original Visible picture:

CRW_0042_VIS.jpg

850nm filtered image:

CRW_0042_IR.jpg

processed with Ned Horning Fiji tool, float:

CRW_0042_NDVI_Float.jpg

color float:

CRW_0042_NDVI_Color.jpg

suggested cfastie lut:

NDVIBlu2Redt.jpg

CRW_0042_NDVI_float_B2R.jpg

The result looks good even I had to hold the two cameras in my hand, these pictures are not with the rig below.

Setup of two cameras.

P1000399.JPG

P1000400.JPG

Questions and next steps

These pictures where taken without changing any exposure setting ofthe camera. For now chdk was only installed for the raw camera capability. I'm wondering if I should take pictured with shutter speed and focal lenght locked for both camera. Or should I leave the cameras to decide what to do by them self.

Next steps: + test with 750 nm IR filter + test with locked shutter speed and focallenght for both camera using chdk + Modify my bixler2 drone to support both cameras with battery.

Why I'm interested

Our most important daily life thing is food and water. I think we need to find ways to help farmers get the best yeild they can while using resources where they are needed, water , firtilizer, pesticide. If you look how much area a farmer needs to cover, a bit of help is needed.


5 Comments

You are going to make a lot of people envious with your lovely crisp images. The NIR photo looks as sharp as the VIS version, so I guess you have a glass IR filter in front of the lens. You should be able to get excellent results with that setup.

The 750 nm IR filter might work better because it should pass more light. Then your exposures can be shorter (or lower ISO, or larger f-stop).

I did a custom white balance on my IR camera (Wratten 87) which eliminates the pink tint. I don't think it makes much difference to the final NDVI, but the pink is just an arbitrary artifact, and black and white NIR photos look cool.

It is not that important to use the exact same shutter speed, aperture, and ISO on both cameras because the NDVI images will be lower resolution (many adjacent pixels will have the same NDVI value). I recorded RAW data via CHDK the first time I did an important flight with the two camera NIR rig, but it was so much trouble I never did it again. When taking aerial photo pairs, the critical thing is synchronizing the shutters and keeping the shutter speeds high. CHDK will allow both of those things (an external timer is also required to synchronize the shutters). I use CHDK to lock shutter speed and the camera selects an appropriate aperture. Generally I use 1/800 for the VIS camera and maybe 1/600 for the NIR camera, but the NIR camera might have a slightly higher ISO.

At the bottom of this page are two look up tables for Fiji which work well for NDVI and images of the color gradient which you can publish with your results. NDVI images have little meaning to most people without a key.

I'm looking forward to the aerial photos.

Chris

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Hello Chirs. I used a glass filter indeed. They where arround 24 euro. I will use a 3DR Pixhawk to trigger the cameras. The cams do not have gps them self. Will inject gps info in the picures, gps comes form Pixhawk. I will try the 750 mm see how that will go and also your idea about shutter speed. Will post the modification i'm making to spuoort dual cam in my bixler 2. Thanks for the info Grayson.

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Nice Job. Some questions, do you have any problem with autofocus in the IR camera?. How do you do to remove IR sensor filter in A2500?

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Very interesting results! I have some questions, so n°1: you bought that filter online, and then simply cutted it so it fit the lens shape in the front of the A2500? May you show a pic of your modified camera? It would be really helpful! for me

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Hallo Albertazzi I bought a 30mm glass filter. Remove the housing of the filter and glue it in front of my cam. See pictures above (No idea how to put pictures in this comment). I try to remove the housing by unscrewing the big ring but did not work. So I use a dremel for it. Also never use crazy glue, 1 second glue super glue or something like that. I know that in confined places the object you are glueing will get a white dust on it. Some chemical reaction, degassing of the glue. I use silicone glue, the one for windows. Works perfect, Next week I will test it in my drone and see if it will hold the vibrations. Check http://publiclab.org/notes/gpenzo/02-16-2014/ndvi-720nm-850nm-filter-two-camera-setup-result for some more results.

Regards Grayson.

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