Tuesday, June 09, 2009

THE SAGA OF THE PERUVIAN GIANT SNAKE (1)

Greg Warner and I are still in dialogue about the possibility of him releasing the photographs he says were taken by the expedition to Peru, and which he says show a snake that may be 130ft long. Greg was interested in what our old mucker Dr Chris Clark had to say the other day and asks:

"Chris, I am interested in your comment about focal point and hight of plane. I have this data, how can we work out size from this? Our scale reference was coming from known sizes of trees in the area."

Over to you Chris...

MORE ON THIS STORY AT:

http://forteanzoology.blogspot.com/2009/06/giant-anaconda-plot-thickens.html

http://forteanzoology.blogspot.com/2009/06/great-snake-or-no-great-shakes.html

http://forteanzoology.blogspot.com/2009/06/more-on-saga-of-peruvian-giant-anaconda.html

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2 comments:

  1. Chris Clark8:25 PM

    Basically, the process of inferring size data from a photograph is called photogrammetry. It depends on the fact that the transformation from object space to image space is linear, apart from lens distortions at the edge of the field. The ratio: (object size)/(distance from object) is equal to the ratio: (size on image)/(focal length). To put it simply, if you know the scale of the image, such as 1000:1, you can measure the size of anything on the image and multiply it by the scale (1000) to get the true size.
    I assume you know the height of the aircraft (how?). You need to the know the focal length of the camera. If you were using a camera with variable optical zoom this will be impossible unless you know what setting you were on (such as maximum). If you know this you may be able to look it up in the manual or, better still, treat it as a measurable quantity. With the camera on the identical setting photograph an object of known size from a known distance: this gives the scale factor immediately. Of course there are complications if you were viewing the object on the slant, since you will need to correct for perspective foreshortening.
    I hope by the way you used a film camera. You are less likely to get sneers about Photoshop if you can produce a negative. Also 35mm film has more image detail than any except the very best professional digital SLRs. If you have film get the negative scanned and digitised; a drum scanner is best. If it was a digital, than I hope you set it to 'low compression', or all the fine detail has been lost!
    I hope this helps.

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  2. yacumama2:21 AM

    Thanks

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