[GRASS-user] Auto-detection of GCP for rectifying image

[Sajid Pareeth]

Hi Johannes


> However, as I want to automatize the step of rectifying/georeferencing I
>> am looking for a way to autodetect these six points in the image. I am
>> thinking of tools like pattern/face recognition that are able to
>> autodetect objects (e.g. eyes, points etc.) and extract their position
>> (coordinates) within that image. I assume these coordinates together
>> with their "true" position could then be used for rectifying the picture
>> using e.g. i.rectify.
>>
>> Has anyone done this or a similar exercise before and can recommend
>> tools and approaches to auto-detect GCP from an image?
>>
>
>
For a similar purpose, to automatize image to image registration, I have
used the library OTB toolbox(https://www.orfeo-toolbox.org/) along with
GRASS GIS.

OTB has a module called homologous point extraction which uses algorithms
like SIFT and SURF.
https://www.orfeo-toolbox.org//Applications/HomologousPointsExtraction.html
After getting the gcp's, I used grass plugin m.gcp.filter to filter out the
outliers, followed by i.rectify.

On a GRASS environment, following is what I used - representative code:

> #GCP extraction using OTBcli_homologous points
> otbcli_HomologousPointsExtraction -in1 input_b1.tif -band1 1 -in2
> reference_b1.tif -band2 1 -algorithm sift -mode full -out OUTB1.txt
> # Making the text file GRASS compatible
> awk '{$5="1\t"$5}1' ${MYTMPDIR}/OUTB1.txt > ${MYTMPDIR}/OUTB1.txt
> # importing the input TIFF files
> r.in.gdal input=${y}_b1.tif output=${i}_b1 memory=${MEMORY}

#Moving the POINTS file to the GRASS group folder
> mv ${MYTMPDIR}/OUTB1.txt ${GRASSLOC}/group/${i}/POINTS
> #GCP filtering
> m.gcp.filter group=${i} order=1 threshold=500 -b
> i.rectify -a group=${i} extension=_rectified order=1 method=nearest --o
>

In this paper (http://www.mdpi.com/2072-4292/8/3/169/htm) , the steps are
explained and main codes are provided in appendix.

Here is a good link on SIFT, I guess it is more apt for your case with
digital photographs. In the above case, we tried on satellite data.
http://www.aishack.in/tutorials/sift-scale-invariant-feature-transform-introduction/

Hope this helps!!

Sajid
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