Well it was an example to show that the required memory is far from huge.

it really depends on how this nodegrid is set up.

lets assume a more or less even nodegrid (more like in an RTS), but where each node can have some positionoffset to adapt it to local conditions.

for the offset 2 bytes (short) are enough, where it says what the offset is to the "normal" gridposition
x,y together 4 bytes
for the height 3 bytes are excact enough, unless you make a scientific simulation
z 3 bytes
---------
Metadata
---------
byte #1 : type of node , where each bit indicates a parameter:
Flags:
1-> is passable,
2-> only narrow chars,
3-> is a locked door,
4-> is in water
etc...

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byte #2: the link flags to the other nodes, (when using my even grid idea) -> if a passble link from my node to the other node is there or not.
to store 8 connection you only need 8 bits then, aligned like this (N=my node)

1 2 3
4 N 5
6 7 8

----------------

altogether 9 bytes per node ...

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ok this is a bit chrunched (on a mobile phone this makes more sense than on a PC), but also an example how to compress the data to save memory.

but 20 bytes per node should be good also.