Thanks soonix - that has got me much further :)

But - I am not understanding how to map the GeoPackage spec to Perl's unpack. I've read perlpacktut although I cannot say I understand much of it. So I've made a start based on your information and after a lot of pondering I think I understand why C is the second and third parts of the template

I don't understand why we start with n
The spec says "byte2 magic = ox4750" - so it's 2 bytes or 16 bits. Why is it n and not v or even s?

Spec 2 says it is an "8-bit unsigned integer" so C fits the bill.

I'm guessing we know 3 is unsigned from looking at the bits laid out in the spec - is that about right?

Now I get completely lost...
There is an int32 between 3 and 4 so that's N or V - but how can we tell? I've tried unpacking with those templates options and get 879493120 and 27700 respectively. I cannot tell from those values which is right.

Next we have 4 double[] envelope
As spec 3 returns 00000101 (5), the envelope consists of 6 values 2: envelope is [minx, maxx, miny, maxy, minz, maxz], 48 bytes so I am using d6 in the template. The returned values don't seem to make sense!

Next is spec 5, the GeoPackageBinaryHeader - I'm took an educated guess this is a but the result didn't seem right so I've used N as it seems to be another flag byte.

Finally we have the geometry which I expect to be text of unknown length. So I tried a100 and A100 but they take me back to the gobbledegook and I cannot work out which one I might need to use.

This is what I have tried... 

use DBD::SQLite;
use Data::Dumper;

use strict;
use warnings;

my $dbh = DBI->connect("dbi:SQLite:uri=file:osopenusrn_202203.gpkg?mod
+e=rwc");

my $tab = $dbh->prepare("SELECT * FROM openUSRN");
$tab->execute;
my $n = $tab->fetchrow_hashref;

my $geo = $n->{'geometry'};

my @test = unpack "nCCVd6C100", $geo;

foreach my $t(@test) {
    print "$t\n";
}

[download]

Am I heading in the right direction here or am I going down a blind alley with my reasoning behind the mapping from the specs to the unpack template?
Is there a simpler way to understand this mapping and work out the correct template?

In reply to Re^2: Geo Package files by Bod
in thread Geo Package files by Bod

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