Yhyjyk

I want to place a node of a tikzpicture depending on a node of a previous tikzpicture.

MWE:

documentclass[a7paper]scrartcl

usepackagetikz
usetikzlibrarypositioning

begindocument

centering
begintikzpicture
node (a) a;
node[below left=of a] (b) b;
endtikzpicture

newpage

begintikzpicture
node[below=of b] (c) c;
endtikzpicture
captionoffiguretest

enddocument

Transforming coordinates, so (0,0) from tikzpicture is put in the desired place of the page and Extract x, y coordinate of an arbitrary point on curve in TikZ indicate that using overlay could be an option, but I actually believe that this isn't the right way to go.

Using an overlay also causes problems to the position of the caption.

If you add remember picture and overlay,remember picture

documentclass[a7paper]scrartcl

usepackagetikz
usetikzlibrarypositioning

begindocument

centering
begintikzpicture[remember picture]
node (a) a;
node[below left=of a] (b) b;
endtikzpicture

newpage

begintikzpicture[overlay,remember picture]
node[below=of b] (c) c;
endtikzpicture
enddocument

you get

Is that what you are looking for or go at least in the right direction?

From the title of the question I am more thinking you may be looking for something like this:

documentclass[a7paper]scrartcl

usepackagetikz
usepackagetikzpagenodes
usetikzlibrarypositioning
usepackagecaption
begindocument

centering
begintikzpicture[remember picture]
node (a) a;
node[below left=of a] (b) b;
endtikzpicture

newpage

begintikzpicture[overlay,remember picture]
path (b|-current page text area.north) coordinate (bshiftedup);
node[below=0pt of bshiftedup] (c) c;
node[anchor=north,align=center,text width=0.9textwidth] at 
(c.south-|current page text area.center) captionoffiguretest;
endtikzpicture

enddocument

That way the continuation really starts at the top of the page, which might be useful if the picture doesn't fit on a page.