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I am currently working on project, where I need to digitise a geomorphological map. I am working in ArcMap 10.5.1. I have an issue with creating new features. When I am digitising an object from the map sometimes, a strange error occurs with the geometry. It looks like after drawing some vertices, something's going wrong, and it seems like ArcMap on its own finished part of the sketch and closed the part of the polygon in very strange way. Here's the example: enter image description here

When I was trying to fix the polygon manually with Edit vertices tool, it didn't worked well, because those long random lines are intersecting the part I have already created and geometry gets really complicated, so it's usually impossible to fix it manually. What's more interesting, during editing the veritices I have realised that the beginning vertex (which is indicated in ArcMap as red) have changed it's location, and now different vertex is indicated that way. Below here is example - I have indicated with yellow arrow the original position of begining vertex. enter image description here

Sometimes, ArcMap is not "finishing part on its own", but those strange lines appear. Also, what's important it doesn't appear with every new feature I am creating, but it is often repeating in the same places of the map (e.g. I was trying to draw a new feature in the same place, when last time this error occured, and it occured again). I have been looking for similar problem, but I haven't found any. I also have tried the classical solution with reinstalling the software but it didn't work. Any ideas what can cause such problem?

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    Polygon boundaries are forbidden from crossing themselves. There are many reasons why a properly digitized line would not close properly (most related to the coordinate reference properties), but there's little chance a line that crosses itself twenty times will produce desirable output. – Vince Apr 30 '18 at 14:31
  • Well, the output you can see on the images, it's the result of the error, not my action on purpose. I am aware that polygons cannot cross themselves, but it's the software which is doing that way. As you can see other polygon on the left side is not crossing itself, because it was made correctly, and ArcMap didn't messed the geometry. My problem is not to force program to make invalid geometry, but the the thing is, it makes the geometry invalid, it's making mess. I hope, that maybe now my problem is more understandable. – Kati93 Apr 30 '18 at 15:27
  • Ok, so in other words, I am trying to digitise a map, and while drawing polygons, sometimes an error occurs, which is resulting in polygons looking similar to the one in the pictures. The thing is, the final polygon won't have any intersections and so, because such thing don't exist in the nature, and it's impossible for geomorphogical units to have such shapes. – Kati93 Apr 30 '18 at 15:46
  • It looks like you were in trace mode and then tried to click some vertices "manually" without changing to the straight line/ two line points tool. – johns Apr 30 '18 at 16:44
  • No, I haven't been using that mode. – Kati93 Apr 30 '18 at 16:56
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It looks similar to what happens when you mistakenly double-click while digitizing/editing a feature instead of single-clicking. Double-clicking indicates to ArcMap that you're done digitizing that shape and that the current vertex is the last one you want to include in the feature, so it automatically finishes the feature by drawing a line between the spot where you double-click and the first vertex you drew.

Trying to clean it up afterwards is usually more trouble than just creating the feature over again. If you're having trouble completely digitizing a feature before this problem occurs, try digitizing it in smaller chunks and assembling them into one feature with the Merge tool afterwards.

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I am not able to answer why ArcMap would be auto completing a polygon and then changing the start point. That's definitely strange. However, hopefully some of the below explanation can help diagnose the issue and help you fix the polygon.

What has happened is that whenever the polygon is closed, and the boundary line crosses itself, the polygon turns into a multipart feature. You may have done this already, but right click one of the vertices and select Sketch Properties. This will open a window that shows the vertex properties of the feature. From here you can evaluate the polygon further. In this case, you can see my example polygon is three parts.

Sketch Properties

Given that the polygon is rather simple, I would just delete it and redraw; ensuring that the sketch is not completed until desired. You can also draw an additional polygon, snapping the intersecting vertex at each error, then merging them together to form one polygon. Ensure that the new vertices are snapped at each intersecting point, plus add them inside of the existing multiparts.

New Polygon Drawn

Select polygons that need to be merged.

Pre-merged Polygons

Drop down the editor menu and select merge. Select either of the polygons in the list, then choose OK. You now have a single polygon that can be verified in Sketch Properties.

Final Polygon

This is a simple example but I hope this helps!

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