# How to create transects from polygon centroid?

I have mapped potential study sites using polygons. What I want to do is create 9 125m transects spaced 75m apart. The center point of the middle transect needs to be the centroid of the polygon. Do you any idea how to do this? I have already created a new shapefile with just the centroid points of each location, but have been unable to figure out how to make transects from that point with it being the center of a line.

Thank you very much!

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Are you using arcgis? If not, which tool are you using? – BradHards Jan 14 '13 at 3:31

## 3 Answers

If you want to create a transect, use DSAS in ArcGIS . You can create transects with the desired length and spacing by use of DSAS .

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In ArcMap, I was able to digitize a single line, then run "Copy Parallel" with varying distances to produce 9 equally offset lines. I ran "Union" on the selected lines producing a single feature. Using the Classic Snap Environment, I defined a Point layer as my snap layer and manually moved the lines, snapping the Center of lines to a point. You can Rotate the line feature too. Hope this helps.

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For an ArcGIS solution, run Mean Center to get the centroid of your polygon. Then Create Fishnet to produce the transects (See the attached screen shot). Essentially, you will want to subtract half the total distance of your transect grid (i.e. 300 m) from your centroid x-coord. Add that value to both the x-coord for the origin and the x-coord for the Y-axis coord. Next, add and subtract 62.5 m to the centroid y-coord and add those values to the y-coord of the origin and y-coord of the Y-axis coord. I've included the appropriate values for the other inputs in the Fishnet screenshot. Finally, clean up the top and bottom horizontal lines with the editor and you've got your transects. You can fairly easily translate this workflow into a python script to automate the process if you have many centroids of polygons.

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