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I am very new at using QGIS. I have the standard installation (no additional plug-ins). I have several polygons and each polygon is formed by a number of segments. What I am trying to do is to connect the polygons with each other with new lines that I create. So for example I have two polygons (left and right are the two polygons) and I want to create a new segment from one point of the first polygon to another point on the second polygon.

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I am using the "catch" function so it catches the points automatically. I create the segment (zoom in BEFORE saving, everything ok) BUT when I click on "save changes" (for the segments layer) both ends of the new segment jump from their position (zoom in AFTER saving, not ok) and are not staying on the polygon-points they were captured on. Why? Also the identification chart of the new segment is empty and doesn't adopt the start and endnode. http://postimage.org/gallery/5fh42dqk/

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I than tried to create a segment and type in manually the StartNode and EndNode but QGIS doesn't connect the two nodes at all. How does this work? Do I need a plugin for this?

Hope you can help me on this. Would appreciate it. Thank you, Tud

EDIT: So i narrowed it down to a round up. When i save the changes i made to the layer (so NOT the project save) QGIS rounds up the coordinates to an accuracy of six decimals. Why is that and how can i disable it? Thank you

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can you add a simple picture of what you are trying to achieve? – vinayan Feb 13 at 9:53
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Welcome to gis.stackexchange. Please read the FAQ and post only one question per thread. – underdark♦ Feb 13 at 9:56
is it anything to do with the crs of the project vs the crs of the layer. – Gerald O'Reilly Feb 13 at 10:59
@underdark i changed the title and will now open a new thread for my second question. sorry. i am using WGS 84 for both. Where can i find out which crs is for the project and which one for the layer? just to make shure. thx – Tudor Feb 13 at 12:27

marked as duplicate by underdark♦ Feb 21 at 8:54

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2 Answers

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If you create a new element, right-click (instead of left-click) on the last node you want to add. Then a textbox lets you add values for the attribute table. These values will not be automatically added to your attribute table!

The Save Button writes the data to the file. That's a different thing than adding objects.

For the snap function, you have to define the snapping radius in Settings -> Options, Digitizing tab, and what layers shall be used for snapping in Settings-> Snap Options.

For the first, I use 10 Pixel to see the cursor snap. If set to Map Units it may not snap at all.

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Hi. Thank your for your answer. I am already using the right-click when creating a new segment, as using the left-click instead just continues creating a second part of that new element and doesn't end the task. Than i get the textbox with the attribute table and i tried adding the right values (such as start and end point/node) but it didn't do anything. I expected that QGIS extends my new segment to the nodes i added in the textbox and also calculates the coordinates and length of the new element because the coordinates of the added nodes are known. But it didn't happen. – Tudor Feb 13 at 12:44
With the snap function i can create the new segment and it snaps the right point i want. I also defined the radius. Everything ok. BUT when i want to save the changes to the layer (so yes, when the data is being written to the file) the new segment isn't snapped on the point anymore (see the pictures). – Tudor Feb 13 at 12:48
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For the first comment: No, Qgis does nothing with the text or numbers you enter. It simply stores them in the table. – Andre Joost Feb 13 at 12:53
up vote 1 down vote

You seem to be dealing with a routable network (judging from your layer names "RoutingSegments"). Please note that you were writing about "polygons" in your question which is rather confusing because the screenshots don't show any polygon layers. I therefore assume you want to add new edges (= new lines) to the RoutingSegments line layer.

Next, there seems to be a misunderstanding about what a GIS like QGIS does when new features are added. Values added in the attribute input dialog (which appears after right clicking) do not affect the geometries at all. (QGIS does not extend segments or anything similar.) Your only way to ensure connectivity is to use the snapping tools correctly.

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Ok, interesting remark. These are in fact routing segments for a routable network. I wrote about "polygons" as the geometrical shape. And your assuming is also right. I do want to add new (routable) edges to my network. And i also had a first success with the combination: "PointLayer" and "SQL" (that i described in my other thread). The fact that the attribute table input does not affect the geometries is now clear to me too :-) Regarding the use of the snapping tools correctly, i still don't understand why after a save the segment end jumps. Have you any idea? thx – Tudor Feb 15 at 15:08
Me again :-) so i have the same problem with the point-layer. I create a new point and position it with the snap tool. I than have the coordinates for x: 10,53387791 and for y: 52,27450957. Now i click on the save button to save ONLY the changes for the point-layer (NOT the project save). The point jumps and i have as new coordinates for x: 10,533878 and for y: 52,27451. It seems like QGIS rounds up the coordinates. But why? How can i change this? Thank you – Tudor Feb 18 at 10:33
follow-up question: QGIS rounds up the coordinates to six decimals when saving layer changes gis.stackexchange.com/questions/52285/… – underdark♦ Feb 19 at 14:54
@Tudor Do you agree that your new question replaces this one? I would close this question if you agree. – underdark♦ Feb 19 at 14:55
yes. I agree :-) – Tudor Feb 21 at 8:38

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