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Topic: Vector filed interpretation

Hello,

I am an undergraduate student starting work on my thesis using Cosi-Corr to assign ice velocities to Himalayan glaciers. First, I am attempting to find velocities of glaciers near Mt. Everest that already have published velocity data to ensure my methods are accurate. I am using two ASTER images that I've orthorectified and resampled using Cosi-Corr. I have correlated the two images and created a vector field using the data from that. I overlayed the vectors onto the images and would now like to extract velocity data from the vectors to assess the accuracy of my correlation so I can then apply this method to other glaciers. Is there anyway within Cosi-Corr or ENVI or maybe some outside way to figure out the amount of displacement that each vector in the field represents? I guess I am more or less confused on how to interpret the vector fields.

Thanks a bunch

Mark

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Re: Vector filed interpretation

Hello again,

I found a thread in the Correlation forum relating to this topic. I created an image using Band Math with the expression sqrt(b1^1 + b2^2) where b1 and b2 are the EW and NS bands of the displacement field but I am still a little confused how this image represents the length of the displacement vector. I understand how the expression should give you the length of the vector but I do not know how to get this length from the image created using the expression in Band Math.

Thanks

Mark

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Re: Vector filed interpretation

What I am looking to produce are images similar to the ones found of pages 4 and 5 of this publication...

F. Herman, B. Anderson, and S. Leprince, "Mountain glacier velocity variation during a retreat advance cycle quantified using sub pixel analysis of ASTER images," Journal of Glaciology, Volume 57, Issue 202, Pages 197-207, 2011

What I do not understand is how to produce the velocity and displacement scale with the maps themselves.

Thanks in advance
Mark

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Re: Vector filed interpretation

Mark,

EW and NS bands give you the displacement, in meters, along the EW and NS directions. Hence, the L2 norm gives you the horizontal displacement of the glacier at every point. If you need the velocity, then simply divide this norm by the time between the acquisition of your images.

Sebastien

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Re: Vector filed interpretation

Sebastien,

One final question. How do you generally go about generate the graphics like in the paper I mentioned above? Can ENVI create graphics such as these with all the annotations (scales, rulers, etc.) or do you like to use another program. Ive attempted to use MATLAB to do some of the graphics with minimal results so far.

Thanks

Mark

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Re: Vector filed interpretation

Mark,

You can use any software you like. You can export the data to Matlab, GMT, or ArcGIS. You can also choose the color bar you like in ENVI, export the image, and finish in Illustrator for the legend. Up to you.

Sebastien

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Re: Vector filed interpretation

Sebastien,

Is there a way within ENVI or maybe Matlab or ArcGIS to create velocity profiles like the ones on page 10 and 11 of the article

D. Scherler, S. Leprince, and M. R. Strecker, "Glacier-surface velocities in alpine terrain from optical satellite imagery—Accuracy improvement and quality assessment," Remote Sensing of Environment, Volume 112, Issue 10, 15 October 2008, Pages 3806-3819

I need to be able to create a profile of the velocity of the glacier along a designated profile line but I am unsure how to draw this line and subsiquently extract the information in a plot like the ones mentioned above. My hope is to be able to create a velocity profile across a line drawn perpendicular to the direction of glacial flow as well as one across a line parallel to the direction of flow.

Also, The vector plot on page 7 of the same paper is something I would like to create. I am able to overlay a vector file on an ortho image in ENVI to get something similar to this but I want to have a legend that relates velocities to the vector lengths in the image. As far as I can tell, the vector lengths that ENVI and Cosi Corr can produce are relative lengths and therefore the lengths do not represent a real physical value. Is there a way to export the data to some other program to create this image?

Thanks,

Mark

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Re: Vector filed interpretation

Hi Mark,

You can produce these plots in many different ways. You can use the profile tool in the ENVI window and export it. You can also export the correlation file in Matlab and use Matlab tools (that's what Dirk did for the profiles you're mentioning). There's also a Profile tool in COSI-Corr that allow you to generate stacked profiles like the ones you can see in this paper:
http://www.tectonics.caltech.edu/slip_h … an2008.pdf

Sebastien