Making a custom index

Agriculture
1

I am using pix4d fields in a project that is looking for a way to identify early signs of a disease in ginger crops. I have taken a series of multispectral images for several weeks in a crop that was deliberately infected. Evidence of the disease was verified and marked through ground observation. When I look at an NDRE index the identfied diseased plants leaves do show up more red than green. The red though is difficult to separate from the ground return colour. What I would like to do is use the RGB image to mask out the ground returns so I just identify green/yellow leaves and then apply that as a “mask”over the red NDRE returns so that it just shows the possibly diseased leaves.

Any good ideas about how to set up a custom index to do that?

Thanks

2

hi,

You could use a masking index as mentioned in this post: Soil and plant masking w/ Pix4Dfields

You should use the first option:

Formula:

(nir-red)/(nir+red) + 0/max(0; (nir-red)/(nir+red) - 0.4)

Note: In this formula, using max() will mask everything lower than 0.4. This value 0.4 can be modified depending on your applications

1600×962 1.01 MB

3

Thanks
I had found that topic and it produced a great result - After I do that mask - How do i put it over an index like NDRE so that I remove all the soil background and then target a leaf return in that spectrum that looks red?

Thanks for your help. I can send some screen shots if that help describe what I am trying to do?

Regards,
Simon

Simon and Amanda Henderson
0422 303 149 // 0456 654 500

4

Ensure you find the new max value in the NDRE layer that needs to be masked.

You could replace the red with rededge in the formula:

(nir-rededge)/(nir+rededge) + 0/max(0; (nir-rededge)/(nir+rededge) - NEWMAXVALUE)