How to determine the number of pixels of a desired object within a digital photograph?

Hi everyone. I want to determine the number of pixels in an object that is part of a digital photo. I use cameras to film chunks of ice breaking away from ice cliffs or tidewater glaciers. I want to know the size of the ice chunk that my digital photographs capture. I know the width and height of the ice cliff in metres already, so I was thinking that I could convert the equivalent of 1 pixel : ? metres - this is really what I want to do. That way every time I see a chunk of ice breaking off, I can pull up the photo, outline the chunk, calculate how many pixels fill the area of ice removed by the chunk breaking off and convert to metres area 2 - thus giving me my size order. If anyone has ideas on how to do this I would really appreciate it, thanks so much community. I was thinking that Photoshop or Fireworks could help me with this, Ricky

Thanks gener7 and normfb.
The Photoshop help measurement page is brilliant and I think I can use the info on that page to work out my iceberg sizes. Check out the attached photo - you are looking at an ice cliff - essentially the front of a tidewater glacier that flows in the ocean. I'm a research student and I look at the process of iceberg calving - this is the term we use to define the sudden breaking away or detachment of ice chunks from glaciers that end in the sea.
I used the lasso tool to crop the area of ice lost as a single chunk of ice detached from the ice cliff. At the same time I had the histogram window open. The histogram showed me how many pixels were contained in the cropped area. What I need to do now is work out what 1 pixel is equivalent to in metres. Soon I will have a digital elevation model for the ice cliff that will offer centimetre to metre scale resolution of the surface topography - so that should really help. The photographs I have are taken every 30 seconds - which is brilliant for documenting how the geometry of the ice cliff adjusts in response to detaching ice pieces and also because the ice is moving. I am a research student in Canada, and at the time I took the photos we measured that the ice cliff was advancing at a rate of approximately 12 metres per day! That's pretty fast. The flow of the ice is a first order control on the rate of iceberg production - you are essentially pushing ice into the ocean where it breaks off. I'm having to learn everything Photoshop from scratch...so please bare with me because maybe some of the photoshop questions I ask are simple to most!
I noticed the "integrated density" function on the help sheet you suggested. This function also provides the square number of pixels contained in the cropped area I select...I guess this is similar to the histogram right?
Either way - i just need the scale conversion to go from 1 pixel : metres or cm equivalent. At that point I think I will have to manually select every single iceberg event from my time-lapse photos and do this the manual way! I have about 800 events to manually digitise and calculate the area for...
Thanks everyone, Ricky

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How to determine the number of pixels of a desired object within a digital photograph?

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