Hi there. I have a picture of a rock with a ruler in it. Quite simply I want to be able to print out this picture with a grid over top with 1cm squared squares relative to the rock. Meaning, I want to be able to tell how big the rick is just by counting the squares. What is the best way to do this? I would expect I need to know the pixel size, then see how many pixels are in each centimeter of the ruler. That would give me my scale. Then set a grid accordingly. This is my first time using Inkscape so I don't know how to do anything... Please Help! Thanks.

There are more ways of doing this, so chosing "the best" way can mean different things.
Apart from that, you will have to measure the ruler on the image and then draw a grid based upon it.
The attached svg contains a way of drawing a basic square for such grid.
Will describe how to construct a full grid on that later on.

Welcome to InkscapeForum!

Hhmmm....I'm not sure the size of the pixel would be relevant....unless I don't quite understand your goal. Assuming you can capture the photo with the ruler precisely parallel to either the x or y axis, you'd only need to align the grid to the cm marks on the ruler. Maybe you have some awesome equipment that would allow you to take the photo with such precision?

But if not....well I'm still not sure if the size of the pixel matters, if your measurement units are cm.... Although, I did learn not too long ago, that Inkscape cm are not the same as real cm. Or actually, I think the problem is not Inkscape per se, but rather your screen resolution. Maybe that's what you mean by pixel size? Someone posted a link to a nice webpage which would make those calculations. Let me see if I can find it....

RATS! I was sure I had saved it, but I cannot find it now And I tried searching the forum too. I found the topic I was thinking of, but it didn't have the link I was thinking of. I'm almost positive I followed a link from the forum to a webpage with a handy calculator. But I surely can't find it now

I'll cross my fingers that whoever posted that before, will see this topic, and post it again

But meanwhile, getting back to the grid, Inkscape has generally 2 varieties. 1 is used as a drawing guide, which can be made visible or hidden as needed, are immovable, and not part of the actual image. There are 2 kinds -- rectangular and axonomic. The other kind becomes part of the image. If it's not so large and/or complex as to challenge your system's RAM, it can be moved. I suppose it could be hidden, using layers, if necessary, or possibly transparency. There are a few kinds of those! Look in Extensions menu > Render.

And related, but perhaps not applicable here, is the Ruler LPE, which basically can add ruler hatchmarks to any path. There are options for several different units, but I can't seem to make any but pixels work. A lot of the LPEs aren't fully developed yet though.

SIGH!! Well, it seems I've written a lot but actually said very little . But at least it will give you a start, until someone who understands the screen resolution issue better than me, comes along

Edit -- Oops, I'm not sure how I missed your reply Lazur URH. I'm curious why you suggest drawing a custom grid. Is there some reason why one of those I mentioned won't work? Ooorrr....are you thinking of allowing for the screen resolution in the grid itself?

Hmm I never used that Descartes-grid extension before.
I was suggesting a way to draw a nearly accurate grid, but if 0,1 pixel accuracy is good enough, then that extension is perfect.
To go with that, simply divide that basic square's length with the number of the measured ruler's marks on a calculator and multiply it as your input data to that extension needs it.