[GIS] How to create transects from polygon centroid

polyline-creation

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!

Best Answer

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.

enter image description here

Related Solutions

[GIS] Determining longest line segment within polygon that passes through its centroid

A radial sweep algorithm will do fine, Duncan. Be aware that the centroid can lie outside the polygon, whence there won't exist any solution in such cases. Notice, too, that this construction is a strange one: whereas the centroid is a global property of the polygon, the line you are constructing is a local property of the polygon in the vicinity of this centroid. The combination doesn't make sense for most geometric or physical analyses where the polygon can possibly be non-convex or non star-shaped with respect to its centroid. (This explains why you won't find it in ArcObjects nor, likely, anywhere else.) You might inquire more deeply of the student to find out what s/he is attempting to do with this construction to make sure it meets its intended purpose.

[GIS] How to create multipart line and polygon with interior ring in arcgisscripting

As adding null point as it was suggested here gave a point with (0,0) cordinates, I searched existing scripts in ESRI repository and I've found another way of separating polygon rings as well as polyline parts.
The solution is based on creating two arrays: featureVertexArray for storing whole geometry and partVertexArray for storing separate rings:

try:
    rows = gp.SearchCursor(inFC) #open SearchCursor on input FC
    row = rows.next()
    outRows = gp.InsertCursor(outFC) #open InsertCursor on output FC

    #create Array object which will contain features vertices
    featureVertexArray = gp.CreateObject("Array")
    partVertexArray = gp.CreateObject("Array")

    while row:
        feature = row.getValue(shapeField) #get the SHAPE field into variable
        vertex = gp.CreateObject("Point") #empty Point object to store geometry

        ##For point geometry there is another way of reading coordinates than for polyline/polygon
        if shapeType.upper() == "POINT" or shapeType.upper() == "MULTIPOINT":
            #this part works right

        else:
            #feature can have multiple parts - first goes iteration through parts
            partNum = 0
            partCount = feature.PartCount
            while partNum < partCount:
                part = feature.GetPart(partNum) #the output is Array of points
                pnt = part.next() #take first point from Array of points

                #iterate through all points in array
                while pnt:
                    #for each geometry create new POINT obj and assign swapped Y, X. Then add vertices to ARRAY
                    vertex.X, vertex.Y = pnt.Y, pnt.X
                    partVertexArray.add(vertex)
                    pnt = part.next()

                    #If pnt is null, either the part is finished or there is an interior ring
                    if not pnt:
                        pnt = part.next()
                        featureVertexArray.add(partVertexArray)
                        partVertexArray.removeAll()
                        #if pnt:
                            #print "Interior:"

                featureVertexArray.add(partVertexArray)
                partNum += 1 #increment part number to run loop

            newFeature = outRows.newRow()   #create new row in InsCur
            newFeature.shape = featureVertexArray       #assign ARRAY filled with points to shape field
            outRows.insertRow(newFeature)   #insert new row                

        featureVertexArray.removeAll() #clear ARRAYS before processing new geometry
        partVertexArray.removeAll()

        row = rows.next()

Best Answer

Related Solutions

[GIS] Determining longest line segment within polygon that passes through its centroid

[GIS] How to create multipart line and polygon with interior ring in arcgisscripting

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