snap intersection circle/line

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snap intersection circle/line

alberich
Hi All,
i have a problem with the snap on intersection between circles and lines to start drawing something. The Cursor will often not snap on these combination. I don‘t know why. Bug? Stupid user? Intersections between two circles works fine, between two lines also. Without this snap (circle/line) simple Euclidean geometry is impossibile :-( I have tried V2.1.3 (Suse Tumbleweed) and V2.2.0-RC1 (Win10). Maybe my circles and lines are a little bit far from the origin (~ 5,000,000; 5,000,000). (In case it‘s helpfull, I have a file to demonstrate the problem and i will upload these if it's possible for a new user.)no_snap_on_intersection_between_circle_and_line.dxf
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Re: snap intersection circle/line

Gary S
Hello  alberich

I saved the file you attached opened it up in V2.2.0-RC1 (Debian), and attempted to draw a few lines with the intersection snap. I had the same problem - I could not snap to the intersection between the line and the circle, at either point.  The other intersections worked fine.  I've used this snap previously and it wasn't a problem...

So, I then started an new drawing, drew a circle and then crossed it with a line at a random location.  I drew additional lines snapping to the intersection of the two and it worked as it should.  I tried a few times, circles drawn as centre/radius, two points and three points.  And crossed each one with a single line.  I then drew more lines to and from the intersections and it all seemed to work as it should.  I don't know why it works for in a new drawing and not in the drawing you provided.  Hmmm...

I moved the entities in your drawing closer to the origin (1,1) and tried again - the snaps work!  Go figure.  I don't know why, but it is as you stated: "circles and lines are a little bit far from the origin".  Perhaps someone with a better understanding of the snaps could offer an explanation...

Cheers,
Gary
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Re: snap intersection circle/line

alberich
Hello Gary
Thank‘s for reproducing this. I will use LibreCAD on a geo-plan from officials and this is in a Gauss–Krüger coordinate system. The objects I’am interested in have this huge offset. I will not permanent moving around the objects. :-( Perhaps it's an other solution or workaround?
Alberich
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Re: snap intersection circle/line

LordOfBikes
Administrator
Definitely, the offset from origin (0,0) and the entity size is the problem.
When the circle at X5,000,000 Y5,000,000 has a radius of 50,000 instead of 5, snap works too.

This is due to the number format in computing, the bigger the difference between operands, the bigger is the loss of precision. In your case the factor from offset to size is about 1,000,000. For a paper size drawing around the origin point, the factor is somewhere around 100 or even smaller.

So I wonder, if the objects are really that small or if this is only a test. Maybe with bigger objects it works as expected.

That's a common problem when using LibreCAD for GIS applications. Some guys started to work on GIS adaption years ago, but I have no idea if there are any results.

There are some workarounds for this problem, but it depends on your needs and preferences which works best.

Armin
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Re: snap intersection circle/line

alberich
Hello Armin,
Thank you. Yes, i will use LibreCAD for GIS application. And the objects are
really that small, for instance a Building 10mx10m but roughly 5,000km away
from the equator. :-) I will move my objects by a defined (and noticed :-)
offset closer to the origin as workaround. Is not a big deal, now i know it's
a common problem.
Thanks all
Alberich