[talk-au] Running stats against GPX files ...

James Livingston doctau at mac.com
Thu Jun 25 12:39:41 BST 2009


On 25/06/2009, at 8:02 PM, John Smith wrote:
> --- On Thu, 25/6/09, James Livingston <doctau at mac.com> wrote:
>> * the DOP has changed
>
> Why would this matter, DOP usually varies, although it is usually  
> pointless recording over 4 or 5, and if you want to 1dp multiple it  
> by ten, so you only really need +/-60 so plenty of room to move in a  
> single byte.

That was mostly so that you don't ever record the DOP unless it's  
changed, because it's not really going to do much over the course of a  
couple of second.


>> There you go, most points now take up 4 bytes :)
>
> in which case you can start doing nibbles instead of bytes, and you  
> might be able to get it down to 2 bytes if you really try.

Yep, and there are a heap other things you can do too.


It all comes down to what you want to use the data for. Are you  
constantly access it? Occasional access? Archival purposes? Read-only  
or are you writing too?

If you're doing geospatial processing, then fixed size records (like  
the 13 byte one mentioned) is good, because you can retrieve any node  
by it's index. If you're archiving the data, what you care about is  
how small you can get the collection of tracks (not just individual  
ones).


> While I agree with your sentiments about GPX, I think some variation  
> of JSON would be more efficient from a programming point of view and  
> potentially just as human readable.
>
> I made a post to this effect on the GeoJSON list, they have a basic  
> spec nutted out, but it's lacking when it comes to most of the  
> valuable information stored in GPX files like time stamps, elevation  
> and hdop.

I quite like XML, but *way* too many people use it when they really  
shouldn't be. You need to ask yourself, do I need the "eXtensible" bit  
of XML? I think anyone who blindly uses XML without thinking why is a  
bit daft, but I also think that of people who will never use it under  
any circumstances, even if it solves their problems.


Anyway...

-- 
James




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