trama de referencia

Talking about mapmaking, I'm trying to translate the following sentence:

cuatro circunferencias directrices, que constituyen la trama o urdimbre de referencia

My attempt:

four directive circumferences , that constitute the reference plot or warp

Thanks for your help.

I think "plot" is correct. However, I think "wrap" refers to the textile industry rather than cartography.
I hope this helps
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