# Plot a point at a user input distance perpendicular to a polyline

I have a straight polyline and the distances in metres of the points I want to plot perpendicular to that line. The points are all different distances and should be measured from a different point along the polyline. Is there a simple way to get these points plotted?

• Can you post a picture and expand a bit? It's a bit unclear what you're asking for. – Goldring Apr 19 '16 at 14:38
• It is a problem of analytical geometry and for the principles, look at How to draw perpendicular lines in QGIS? – gene Apr 19 '16 at 17:07

## 1 Answer

Linear referencing is the way to go for this problem. If the Route measures are based on the length of the line then all you need is a table that contains the Route Name, the distance along the line increasing from the end where you start your measures, and a distance offset. By using the Make Route Event Layer tool or table context menu option you can create a layer of points at those positions on the line with the offset specified. Offsets have to be positive or negative to offset to either the left or right of the line. There are settings that can change which side of the line uses positive offsets and which uses negative offsets. Since your line is straight, that is the ideal use of this approach.

However, there is a limitation when you apply this and other techniques to lines that are not straight. The perpendicular offset is always relative to just one pair of vertices, never three or more vertices. So if a position occurs at the point of angle deflection between three vertices, the perpendicular offset will only consider two of those vertices and create a 90 degree angle relative to those vertices. It will not derive an average angle using three vertices that would result in an intermediate angle between the three vertices. Also if the offset would be affected by the curve created by a buffer of that offset distance near changes in the line's bearing, that is not considered by the tool. Outside of a tool I created for my personal use, I have not seen any tool that considers more than two vertices when determining a perpendicular angle to a line.

• I tried using this tool and it succeeded but there are no points plotted. It has made a sheet with the distances and offsets I specified but no points plotted. In the attribute table for the layer created there is a column that says LOC_ERROR and for each point it says route not found. Is it acceptable to use an excel table? – Dylon Grogan Apr 20 '16 at 8:24
• Please disregard the above comment, I just had the wrong information in the route identifier field – Dylon Grogan Apr 20 '16 at 8:49