Creating an expanding square for search and rescue

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01-28-2016 08:00 AM
SusanZwillinger
Occasional Contributor

Here's something to think about for fun:  Calculating an Expanding Square (Microsoft Excel) The link provides a download for a spreadsheet that will calculate the points for a search and rescue effort that starts at a center point and then expands from that location in a spiraling rectangle. (It says that it creates an expanding square, but that only sort of works near the equator; for other areas it is a rectangle.) Once you download the spreadsheet, you can "unprotect" the worksheet (there is no password) so that you can format the table array to bring the data into ArcMap.  If you add a column for "Line" and calculate all of the rows to be "1", you can use the Data Management Tools-->Features-->Point to Line script to convert the points into a line using the "Seg" column as the sort order.  The spreadsheet formulas have some obvious issues, but it is a quick approximation of what someone might need.  If you were going to improve the solution in ArcGIS so that it was a more accurate expanding square, how would you do it?

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DarrenWiens2
MVP Honored Contributor

The example uses an approximation of geographic coordinates. It's kind of cool how they set up the even/odd routine. You can (perhaps) do better letting ArcGIS handle the coordinates through a projection (spatial reference).

In Python:

>>> x = 0 # start x coord

... y = 0 # start y coord

... step = 10 # distance between lines

... points = [] # points list container

... line_array = arcpy.Array() # line vertices container

... sr = arcpy.SpatialReference(3005) # spatial ref

... for i in range(100): # we're going to make 100 points

...    if i%2: # check even/odd

...        if i/2%2: # check 1/2 even/odd

...            x += step * int(i/2) # increase x

...        else:

...            x -= step * int(i/2) # decrease x

...    else:

...        if i/2%2: # check 1/2 even/odd

...            y += step * int(i/2) # increase y

...        else:

...            y -= step * int(i/2) # decrease y

...    point = arcpy.Point(x,y) # create point

...    points.append(arcpy.PointGeometry(point,sr)) # add point to list

...    line_array.add(point) # add location to line vertices

... line = arcpy.Polyline(line_array,sr) # create line

... arcpy.CopyFeatures_management(points, r'in_memory\points') # write points

... arcpy.CopyFeatures_management(line, r'in_memory\line') # write line

SusanZwillinger
Occasional Contributor

Wow, that was fast!  The code runs fine, but I'll have to play around with the spatial references. 3005 is for NAD_1983_BC_Environment_Albers so if you use latitude and longitude coordinates as the x,y input, it is not going to work.  What did you use as your inputs?

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DarrenWiens2
MVP Honored Contributor

The spatial reference is set in the line: sr = arcpy.SpatialReference(3005)

3005 is the factory code/authority code/EPSG code for BC Albers. Change it to the CRS of your choosing, but keep in mind that the units of 'step' will be determined by the CRS. If you input a geographic CRS, then the 'step' will be in units of degrees.

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