Question about clustering algorithm(projection)

Topics about the trajectory clustering program for HYSPLIT.
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mc2128
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Joined: March 4th, 2014, 9:06 am
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Question about clustering algorithm(projection)

Post by mc2128 » July 21st, 2015, 4:34 am

I was looking for the algorithm for the clustering and found out that the clustering is done after projection.
I'm just curious that why clustering process should be done after projection process. Is there any reason for that not using the great circle distance or something?
Because it would be just hypothetic situation but if trajectories are starting from equator to near polar then clustering would be different between each mercator, polar and lambert projection.

I used this make_traj.sh file to test them and trajectories from this sample file showed different clustering for different projection.

*Added, I thought file wass uploaded but it wasn't. I upload the make_traj.sh file.
make_traj.zip
make_traj
(604 Bytes) Downloaded 104 times
Last edited by mc2128 on July 21st, 2015, 9:55 pm, edited 1 time in total.

barbara.stunder
Posts: 272
Joined: November 9th, 2012, 4:23 pm
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Re: Question about clustering algorithm(projection)

Post by barbara.stunder » July 21st, 2015, 7:29 am

Using a projection avoided longitude issues using the great circle distance for a set of south polar trajectories so the program was changed to a projection.

The 'auto' projection sets the projection based on the trajectory origin latitude - polar with absolute values of latitude greater than 55, mercator less than 25, otherwise, lambert. There probably can be some differences in the cluster results near the borders. How large are the differences you are finding?

mc2128
Posts: 7
Joined: March 4th, 2014, 9:06 am
Registered HYSPLIT User: Yes

Re: Question about clustering algorithm(projection)

Post by mc2128 » July 21st, 2015, 9:51 pm

Thank you for your answer.
The difference was not that large. It was just curiosity.
If I using trajectories from the file that I uploaded on original message,
they all have 6 cluster, but as you can see in the attached file, for the lower number of cluster the clustering is little different. (polar has 3, but lambert or mercator has 4 cluster instead)

*Here I attach clustering file and image for each projection method.
all the trajectories used were same. (other specific images are on the "iamge.zip" file)
clusmean1_3.ps.png
Polar
clusmean1_4.ps.png
Mercator
Attachments
image.zip
other image files
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barbara.stunder
Posts: 272
Joined: November 9th, 2012, 4:23 pm
Registered HYSPLIT User: Yes

Re: Question about clustering algorithm(projection)

Post by barbara.stunder » July 28th, 2015, 9:23 am

In looking at your script make_traj.sh, you are arbitrarily making trajectories by giving a series of latitudes and longitudes, writing it in the format for the HYSPLIT cluster program. I've never seen any atmospheric transport due north/south like that from the equator to the north pole. Please see the sections on Trajectory calculations and statistics at http://www.ready.noaa.gov/documents/Tut ... index.html.

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