Tuesday, April 29, 2014

Line Modification

Tips for creating a line - like dropping points, freehand drawing and undo - were previously covered in this blog post.  Options for modifying existing lines have slowly been growing, and I think it's time to take a detailed look at them.

All of these options are accessed by right clicking on an existing line.  Vertex dragging is also enabled when clicking the edit icon in the left bar.

the right click context menu

The first option I want to cover is "Clone As".  This allows you turn a line into a polygon, add a styled line that matches a polygon border, or simply backup an existing line before editing or resampling:
right click -> Clone As -> Line.  Done.

Vertex dragging has been around for a while but it's worth a quick look.  Right click - > Modify -> Drag Vertices.

At this point each vertex along the line or polygon gets a draggable white circle.  Midpoints between vertices also get a partially transparent circle, which you can drag to create a new vertex.  To delete a point, right click on it and choose Delete Vertex.

When you're done modifying the line, right click on it and chose Save Changes - or, if you made a mistake, Discard Changes.

Sometimes it's useful to break a line in two - say to delete a bunch of points at once, or to get around a GPS' limitation on the number of points per route.  To do this, simply right click on a line at the point where you want it split, and choose Modify -> Split Here.

The opposite of splitting is joining.  To join lines,  one of the two must have a label.  Right click on the unlabeled line and choose Modify -> Join Lines.

A dialog will come up asking what line you want to join with.  If the two lines don't touch, CalTopo will automatically pick the best way to connect them.

Drawing a freehand line creates a lot of points.  This can exceed the limits of your GPS, resulting in only getting part of a route.  It also creates jitter, since no one has perfectly steady hands.  You can work around these problems by resampling a line, reducing the total number of points.

You can shrink a line down to a fixed number of points, or to create points at a fixed interval like 500 feet.  You can also optionally add a marker at each resampled point, in order to load them as waypoints onto a GPS.

Finally, you can reverse the order of a line so that directional styles will point in the opposite direction.


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  2. can you merge two lines at a specific point, all while keeping the two different named lines separate?

    1. I'm sorry, but I'm having trouble trying to figure out what you're asking.

      FWIW, for any modification, you can leave the original intact by right clicking, choosing Convert -> Copy as (line/polygon/etc) to duplicate the line, and then modifying one of the two copies.

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