Author Topic: Region Size for NAC Image Strips ?  (Read 3162 times)

bandred

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Region Size for NAC Image Strips ?
« on: April 17, 2011, 09:30:47 pm »
Hi,

when I first arrived and was just pushing buttons to decide if I needed to read the manual, I'm sure I remember seeing the size of the area covered by the NAC image strips, but I can't find it again now.  I seem to recollect that it was 2km wide by 20 km high.  Can anyone confirm that this is correct ? or point me to the correct values if not please.

many thanks,

Mike

astrostu

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Re: Region Size for NAC Image Strips ?
« Reply #1 on: April 18, 2011, 01:45:07 am »
Mike - the rough answer is that NAC strips are about 2.5 km wide.  The length varies but seems to generally be around 10x the width.  Keep in mind that the camera does not look straight down to capture the image, but it is often looking at the surface at an angle.  This has to be corrected for and while the original image if taken straight down may have been 2.5 km by 25 km, once you properly project it this will change.

It is more correct to state a field of view, which is 2.86° wide covered by a 5000 px line-aray CCD.  I'm fairly sure that it's a pushbroom model meaning that it can actually record images of a certain width but of any length (length limited for practical reasons) with a continuous readout to give the image.  So some are shorter and some are longer.

bandred

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Re: Region Size for NAC Image Strips ?
« Reply #2 on: April 18, 2011, 12:13:35 pm »
Stuart - Thank you for taking the time to reply.  I'm fairly sure I follow what you said.  But, just in case; - I think you are saying that the 'camera' is a 5,000 wide pixel line-array that typically scans approximately 50,000 lines of data to provide the NAC images.  If the camera is looking straight down, from an implied reference distance/alititude, the width of the terrain covered would be 2.5 km.  If the actual distance is greater than the implied reference distance, then the width of the observed terrain will be greater than 2.5km, given the fixed field of view.

My reason for asking the question is that I have been downloading NAC image strips to examine in GIMP.  I'm essentially, looking for some way to estimate the size of some of the objects that I am seeing. It seems from your information that I can at least get a first order estimate by assuming that one pixel is equivalent to 0.5 metres, which should be good enough for what I am doing at the moment.

One last question though, if you don't mind.  I see reference to 'scaled pixel width' and 'scaled pixel height' in the image details associated with the NAC images.  Do these have any affect on the images displayed ? ... I'm currently assuming that the images are raw data.

Thanks and take care,

Mike

astrostu

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Re: Region Size for NAC Image Strips ?
« Reply #3 on: April 18, 2011, 04:32:51 pm »
First two paragraphs are basically correct.

One last question though, if you don't mind.  I see reference to 'scaled pixel width' and 'scaled pixel height' in the image details associated with the NAC images.  Do these have any affect on the images displayed ? ... I'm currently assuming that the images are raw data.

The raw data actually do not have "square" pixels.  Yes, on a computer they are square, but the pixel width corresponding to meters on the ground is not the same as pixel hight.  So you would actually need to shrink the width (I think it's width) by that scaling number to get them to be square.  If you're computer-savvy, comfortable working via command line, are REALLY interested, and have about 50 GB of spare hard drive space, you can actually install ISIS which is what is used to process the raw images and convert them into a proper, georectified (square pixels), map-projected product.

bandred

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Re: Region Size for NAC Image Strips ?
« Reply #4 on: April 18, 2011, 05:24:38 pm »

 If you're computer-savvy, comfortable working via command line, are REALLY interested, and have about 50 GB of spare hard drive space, you can actually install ISIS which is what is used to process the raw images and convert them into a proper, georectified (square pixels), map-projected product.

Stuart -  Thank you for the reply.  I've had a quick look at the ISIS link and, as much as I enjoy a learning experience, have decided that I am geting too old to take that one on :)  If I find any interesting images I'm sure somone will do the correction, if they feel it warrants it  .... as you have already demonstrated  :) Thanks for that by the way.

take care,

Mike