Set scale to draw on image background

[copyrightx]

I have a google satellite image with scale shown in the image. I want to trace a drawing of a parking lot onto this image, but I need to set certain distances (eg. 9 feet for width of a parking space, 4 inches for parking lot line width, etc.). For scale, the image shows a line that is 20 feet long, but how can I tell QCAD that I want that distance to be 20 feet for everything in my drawing?

[andrew]

I'm afraid, you'll have to do some simple math:

1. Measure the distance you know to be 20 feet with
Info > Distance Point to Point
2. Copy or write down the result
3. Scale the image with factor 20/result (that's 20 divided by the result measured from step 1)
You can use Modify > Scale to scale the image.

You might also find this related thread helpful:
viewtopic.php?f=32&t=3305

[copyrightx]

Thanks, Andrew.

I found that the Modify->Scale menu item was grayed out, so I followed the guidance in the thread you cited:

Select the jpg (upper right corner works best),
Start the Scale tool,

And I inserted my quotient (20/79.11663206 = 0.25279134714471313) into the "Factor x" field of the "Scale Options" dialog box that popped up when I did so.

But I don't get the result I expect (perhaps because I'm confused about what I should expect). I would think that after doing this scale operation, the units of my drawing on the axes would indicate that the line in question (which I think I just defined as 20 feet) is 20 feet in length by the axis scale numbers (rightmost point of line - leftmost point of line = 20). But that's not what I'm seeing. Instead, when I measure the 20 foot long line, it measures as about 100 feet.

Any idea what I did wrong?

[andrew]

Can you attach your drawing file with the bitmap? Thanks.

[copyrightx]

Through much trial and error, I solved this problem. For anyone who experiences something similar, I honestly think that no amount of writing will help me explain how to solve it. But I'll attempt by simply saying that you should do multiple attempts at scaling as is described in this thread. Do this repeatedly while studying very carefully the axis' measurement scales. I think if you do this, then you too (like I eventually did) will understand how to sort this out. Sorry I can't do better at explaining than that, though.