How to automate selecting specific xyz map data from an existing xyz data set?

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BlkHoleSun 2019-1-13

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回答： BlkHoleSun 2019-1-18

I have a bathymetry map that provides a 1 meter resolution. However, I don't want all of the map data. I want a 1024X1024 meter section of the map so that I can run comparrisons. The xyz map data I have is lat/long/alt and I'd like to automate the process and be able to populate a 1024X1024 matrix with z data.

I have attached a link to the xyz file since it is too large and a .jpg file so that you can see what the map looks like. I've also drawn a blue box around the desired location.

http://seafloor.otterlabs.org/SFMLwebDATA_mb.htm#MONT

This is a painful process to go cell by cell so I appriciate all help.

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BlkHoleSun 2019-1-15

I've made some good progress so far. I've been able to get the lat/lon values and create a box around what I want. I was attempting to extract the data by finding the values that are > or < the lines I made around the box. That doesn't seem to be working since the find function doesn't like non-constant threshold values. Any thoughts or advice on extracting the desired data?

Walter Roberson 2019-1-15

A question would be how accurate the boundaries have to be. The map spacing is in physical meters, but imposing lat/long boundaries implies boundaries that are rectilear in physical meters. Your source about 7.3 km by about 6.9 km, which I figure is large enough that the curvature of the Earth would several few meters off of the upper left and upper right if the lat/long boundaries need to be strict.

I would tend to suggest that you just translate the lat/long boundaries at the southern edge and use those to select a rectangular portion of the data instead of worry about the curvature.

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BlkHoleSun 2019-1-18

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Walter and KSSV,

Sorry it's been a while. I figured it all out. I first determined the slope of the box I wanted and the top left point (could have pick any point). I then used algebra to compute the other corners of the box. I then did a linspace to make 2 sets of parallel lines with 256 points. I then created a for loop to connect the two lines using linspace again for 256 points inbetween the two lines. This resulted in a 256X256. Next, I had to get z data over. I used the reshape function to get my 256X256 into a single column, then used the griddata function for the z values and used reshape to get it back into a 256X256. Now I'm on to calculating Boltsman and Shannon entropy measure for comparrison! Thanks for all your help!

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