Why when I do an intersect analysis on a point feature class to a line feature class I only get ‘one’ point result?

The Case of the Disappearing Points: Why Your GIS Intersect Isn’t Playing Nice

Ever run an Intersect in your GIS software, expecting a bunch of points where your point layer meets your line layer, only to get…one? Yeah, it’s happened to the best of us. It’s like, where did all my points go? It can be super frustrating, but don’t worry, there’s usually a pretty straightforward explanation.

Basically, it boils down to how GIS software thinks about points and lines, and how the Intersect tool is set up to handle their interactions. A point, in GIS terms, is basically a zero-dimensional thing. A line? One dimension – length. When a point sits exactly on a line, the Intersect tool sees that as a match. But here’s the kicker: often, the tool is set to favor the feature with the smallest dimension. Point wins! So, you get a point layer out.

Okay, but why just one point? Well, think about it this way: the Intersect tool isn’t just looking for points near a line. It’s looking for points smack-dab on the line. If you’ve got a bunch of points all lined up on the same line segment, the tool might just give you one point to represent that whole cluster. It’s like it’s collapsing them all into one.

Let’s break down the usual suspects and how to wrangle them:

1. Point Overload (aka Coincident Points): This is the big one. You might have a bunch of points that are practically on top of each other. If they’re close enough that the software thinks they’re the same point, the Intersect tool will treat them as one.

• The Fix: Hunt down those duplicate points! Most GIS packages have tools to sniff out and nuke identical geometries. Look for something like “Find Identical” or “Delete Identical” and run it before you do your Intersect. You can also tweak the XY Tolerance to tell the software how close is too close.

2. Software Being Picky (Settings and Tolerances): The Intersect tool follows the rules set by your software’s environment settings. XY Tolerance is a big one, and whether the Output has M or Z values can matter too. If the XY Tolerance is too tight, points that are almost on the line might get ignored.

• The Fix: Peek at your software’s environment settings, especially that XY Tolerance. Bumping it up a tad might grab those “close enough” points. Just don’t go overboard, or you’ll end up with inaccurate results. And if your data has M or Z values, make sure those are being handled correctly.

3. Snapping Snafus: Ever digitize a map and think you snapped perfectly, but then zoom way in and see a tiny gap? That’s a snapping error. Points that look like they’re on the line might actually be a hair off.

• The Fix: Be a snapping ninja when you’re creating data! If the data’s already made, try the “Snap” tool to force those points to lock onto the lines before you run the Intersect.

4. Coordinate Chaos (Data Projections): If your point and line layers are using different coordinate systems or projections, things can get wonky. It can introduce subtle distortions that mess with your intersection.

• The Fix: Make sure everything’s speaking the same language! Both layers need to be in the same coordinate system and projection. If not, use the “Project” tool to get them aligned before you intersect.

5. Tool Quirks (Software-Specific Parameters): Sometimes, the Intersect tool itself has hidden knobs and dials that affect how it works. These parameters can change how it deals with points that are close together.

• The Fix: RTFM! (Read The Fine Manual!). Check your GIS software’s documentation to see what parameters are available for the Intersect tool. Experiment a bit to see if they make a difference. Look for anything about handling coincident features or keeping attributes.

So, next time you’re scratching your head over disappearing points, remember these tips. Take a good look at your data, double-check your settings, and don’t be afraid to experiment. With a little detective work, you’ll get your Intersect working like a charm. Trust me, I’ve been there!