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A Note About UTM vs LAT/LON

For some reason people seem to get wrapped around the idea that data projected in UTM is superior to unprojected geographic data (i.e. latitude and longitude).  Granted, if you have a desperate need  to make a paper chart, lay it on a table, and measure distance with a ruler then UTM is your baby.  If you want to make ECDIS systems then it’s probably a good idea as well because it’s nice to be able to glance at a map on a display and know that distance is the same in any direction.

The PFMABE suite of software is not designed to make pretty pictures or output products (except in a very limited sense).  It is designed to allow you to quickly and easily look at your point data and invalidate (or re-validate) fliers and divers.  That’s pretty much it in a nutshell.  In order to do this we don’t need to use projected data.  We need some way to organize the data by area.  That’s what PFM was designed for.  PFM will handle projected coordinates but it isn’t necessary for what we’re trying to do.  That’s why PFMABE doesn’t use projected coordinates.

If you look at the PFM data structure you will see that it consists of geographic bins that are some number of (nominal) meters square.  The way we do that is to calculate the size, in latitude and longitude, of the bin at the center of the minimum bounding rectangle of the PFM area and then use that throughout the PFM.  This is the point where most people start to panic.  They run around screaming “Oh dear lord save us, we’ve made a bin!  It’s not accurate to within a millimeter! What shall we doooooooo?!”  DO NOT PANIC.  The bins are not used for any purpose other than to show you where there are anomalies in the min/max/standard deviation surfaces, keep track of what’s been changed, and give you a link to the data points.  The data points are stored in latitude and longitude (sort of, you don’t really want to know but the resolution is 1/4095th of the bin size).  This is the second place where most people start to panic.  “AAAARRRGGGHHH, the resolution is only 1/4095th of 4 meters!  We’re all going to die!!!  Wait, what?”.  I say again, DO NOT PANIC.  The position and Z values are not used for anything other than to allow you to see the data points relative to each other in 3D space so that you can decide what you want to invalidate.  The points also allow you to link back to your input file/record/sub-record so you can do cool things like display the associated waveforms for LiDAR data while you whip around through the point cloud like Captain Kirk through a Klingon mine field.

Almost everything in PFMABE is designed to allow you to mark data points as invalid (or valid or reference or selected or suspect) and then save that validity data back into your original input file.  It is not designed to allow you to modify position or Z.  In fact, you can really think of PFM as a highly specialized spatial data base.  When you’re done with it you can throw it away because all of the editing you’ve done is back in your input files.

Now, all that being said, you can extract data from the PFM to make things like GeoTIFFs, LAS/LAZ files, ASCII files, or BAG files but that’s not the major thrust of PFMABE.

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