on December 29, 2022

Creating series of lines offset from starting points (ArcGIS, arcpy)

Hiking & Activities

Creating Lines Offset from Points in ArcGIS: Ditch the Drudgery with ArcPy

Ever needed to whip up a bunch of lines, each starting from a point but nudged over a bit? Maybe you’re mapping utility lines, modeling stream buffers, or just trying to get creative with your cartography. Whatever the reason, ArcGIS has your back, but sometimes the manual tools just don’t cut it, especially when you’re dealing with a mountain of data. That’s where ArcPy, the Python library for ArcGIS, comes to the rescue. Trust me, it’s a game-changer.

So, what’s the big deal? Well, the trick is automating the creation of these offset lines. We’re talking about lines that sprout from your points but are deliberately shifted by a certain distance and direction. This means getting down and dirty with feature geometry, something ArcPy handles like a champ thanks to its arcpy.da module and those nifty geometry objects.

Now, there are a few ways to skin this cat. Here are some strategies I’ve found useful over the years:

  • The “Trial and Error” Line Generator: This one’s a bit brute-force, but it works. You basically fire off random lines from your starting points and then check if they run into anything they shouldn’t, like a shoreline. If there’s a collision, you just regenerate the line until it behaves. It’s like teaching a stubborn puppy to fetch, but hey, sometimes it’s the only way.
  • The Buffer-to-Line Shuffle: This method involves turning your original line into a polygon using a buffer. Then you convert the buffer to lines, extract the vertices as points, buffer those points by a tiny amount, and split the lines at each vertex. Sounds complicated? It is a bit, but the final step involves using a spatial join to attribute the new lines with direction.
  • The “Offset” Tool (ArcGIS Pro Only): If you’re rocking ArcGIS Pro, you’ve got a secret weapon: the “Offset” tool. It lets you create features at a specific distance along a baseline, with a perpendicular offset from the line. You can use it interactively, but the real power comes from scripting it.
  • The Direction Distance Construction Trick: I’ve used this one for quick edits. You can create a point at an offset from an existing one using the Direction Distance construction tool. With point snapping turned on, this tool allows you to snap to the existing point while setting the direction and distance.

    • Okay, let’s get our hands dirty with some ArcPy. Here’s a basic recipe to follow:

  • Summon ArcPy: Kick things off by importing the arcpy module. It’s like saying “Open sesame!” to the ArcGIS geoprocessing kingdom.
  • Lay Down the Ground Rules: Define your input point feature class, where you want the output lines to go, the offset distance, and anything else that matters, like the field holding your bearing information.
  • Grab Your Magnifying Glass (Search Cursor): Use arcpy.da.SearchCursor to stroll through each point feature in your input feature class. Think of it as a digital detective, examining each point one by one.
  • Snag the Goods (Geometry and Attributes): For each point, grab its coordinates (X and Y) and any other juicy details, like a bearing or angle.
  • Crank the Numbers (Calculate Offset Point): This is where the math comes in. Based on your offset distance and bearing, calculate the coordinates of the new endpoint for the line. A little trigonometry (sine and cosine) will get you there.
  • Build the Line (Create Geometry): Create arcpy.Point objects for both the original point and the calculated offset point. Then, create an arcpy.Array, toss in both points, and finally, create an arcpy.Polyline object using that array.
  • Plant the Evidence (Insert Feature): Use an arcpy.da.InsertCursor to drop the new line feature into your output line feature class.
  • Clean Up Your Mess (Delete Cursors): Always a good practice. Delete those cursor objects to free up resources.

    • Here’s a snippet of code to get you started:

    python

    You may also like

    Field Gear Repair: Your Ultimate Guide to Fixing Tears On The Go

    Outdoor Knife Sharpening: Your Ultimate Guide to a Razor-Sharp Edge

    Don’t Get Lost: How to Care for Your Compass & Test its Accuracy