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Thread: Calculate point location from offset observer

  1. #1
    David Gordon
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    Default Calculate point location from offset observer

    I'm conducting a forest bird survey in which I need to precisely record locations of individual birds. It's impossible to use a GPS to directly find the coordinates for each sighting because of the steep terrain and forest canopy. But I do have UTM coordinates for many control points across my study site. From these controls, I can get an azimuth and distance to observed birds using a compass and range finder. So, is there a tool, program, or calculation I can use to find the UTM coordinate of a bird from the coordinate of the observer, plus distance and azimuth estimates? It would be perfect if I could do this as a calculated field in my attribute table. Any help would be very much appreciated!

    I'm working with ArcGis 9.3.

    Dave

  2. #2
    Hardolph Wasteneys
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    Default Re: Calculate point location from offset observer

    Dave,

    As an interim method, i.e. clunky, you can locate individual Bird sites using the "direction-distance" tool in editing functions for drawing polylines and locating vertices. This is activated by snapping the sketch tool with a polyline FC to your control/reference point then right clicking for the pulldown menu or CTRL- G and entering the direction-distance data. Then you would have to snap a point to the end of the line that is drawn. This will work very precisely, and could be sped up by entering the direction and distance info in fields for the reference point so you can see them in the attribute table then draw all the lines before switching to the point feature class to add points at the end of the lines for your birds.

    Alternatively, you can do this with the direction-distance editing tool in a point feature class only, (looks like a circle with a line through it in the 9.3 editing tool pulldown). First snap to you reference point and hit A to enter an azimuth, snap to the same point again and hit D to enter the distance, then hit enter to make a new point.

    I think I prefer the polyline method; having to snap back to the same point twice is a bit confusing and drawing the polylines will create a verifyable record of how you located the points.

    With the points located you can output the coordinates in new fields in your point feature class using Calculate Geometry.

    Another way that runs a bit outside ArcMAP, I guess you could export your GPS reference points to a spreadsheet then add your azimuth and distance and calcuate trigonometrically (add D*sin (az in radians) to the X coordinate and D*cos (az in rad) to the Y for new X-Y positions then return that table to ArcMAP as an ADD X-Y Data function from the Tools menu using the new coordinates. You could do the calculation in ArcMAP but you would have to reinput the X-Y table anyway.

    good luck,

    Hardolph
    Hardolph Wasteneys Ph.D., P.Geo.
    Geologist
    Vancouver Island

    hardolph@bravewolf.com

  3. #3
    David Gordon
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    Default Re: Calculate point location from offset observer

    Good ideas Hardolf! I'll try using the direction-distance tool in editing. I hadn't thought of that. Thanks!

    Dave

  4. #4
    Darren Wiens

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    Default Re: Calculate point location from offset observer

    I can't remember exactly what software I was using, but I know for sure there are programs out there, for free, that will take all of your bearing/distance measurements and output a point shapefile. I think we may have been using Locate, it looks about right.

  5. #5
    David Gordon
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    Default Re: Calculate point location from offset observer

    I finally got around to doing this analysis using the trigonometric approach suggested by Hardolph. I transferred all the data from my attribute table into Excel, then set up the functions described by Hardolph to calculate the bird locations, then brought it back into Arc as an XY event layer. Worked great! Thank you!

  6. #6
    Ryan Trimbath
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    Default Re: Calculate point location from offset observer

    How did you collect your direction data? I would assume that you used compass direction, with North being 0, and following compass degrees. The problem with using radians, or Trig functions seems to be that the 0 in Radians is a compass 90, or East. Additionally Radians increase counter-clockwise as apposed to clockwise when using compass degrees. I am doing a similar task and I cannot figure out how to compensate for this offset. Did your points line up correctly?

  7. #7
    Ryan Trimbath
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    Default Re: Calculate point location from offset observer

    Maybe the problem is related to my PCS NAD_1983_UTM_Zone_17N

  8. #8
    Ryan Trimbath
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    Default Re: Calculate point location from offset observer

    It works but the degrees you need to subtract your bearing from 450.

    Rad=((450-degrees)*pi)/180

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