I've been asked to map point data that was collected in the field by manually writing down the coordinate from a Garmin NUVI 1490 LMT. They were put into a spreadsheet and sent my way which I was able to convert from DDM to DD in Excel. I've tried a bunch of different ways to import the points into ArcGIS using Add XY data with the same result (see attached image of street centerlines and points). The street centerine data is what I'm hoping they'll line up with (they're locations of street signs).

At first I suspected the projections I was using but after much experimentation I think it may be the conversion formula I used (found here: http://www.ozgrid.com/forum/showthread.php?t=155776).

Can anyone offer any advice as to where I've gone wrong and set me straignt?
enter image description here

  • Are you sure the coordinates in the original spreadsheet were degrees with decimal minutes, and not degrees, minutes, and seconds? – Dan C Apr 14 '14 at 19:00
  • Yes I'm sure, they looked like this: (40 19.579) – Eric Apr 14 '14 at 19:19

the equation that you are pointing to seems to be correct. (degree + minutes/60 + seconds/3600). The error could come from a wrong datum. Data from a GPS are usually in WGS 84, but some GPS make an internal conversion into a local datum. Checking the datum is thus the first thing to do.

Also note that ArcGIS is projecting on the fly, but it is not applyig the datum transformation automatically. In the data frame properties > coordinate system, you should check that a transformation is applied between the datum of your lines and the datum of your points.

The quick and dirty way to solve the problem is to move your points with an affine transform using the "spatial adjustment toolbar" and 4 or 5 points that you can identify at road intersections.

The more rigourous method consist in adding your XY coordinates as point without specifying a datum, and changing the geographic coordinate system of your map until you get a match (the lines, which are georeferenced, will be projected on the fly, and the points will be displayed in the coordinate system of the map, so when the coordiante system of the map is correct you should get a match). Of course, every time you change the datum, you need to set a transform otherwise nothing will change.

  • I realize there are a lot of variables here but I appreciate your feedback, I had come up with most of this on my own but it's a nice sanity check. Thanks! This forum is awesome. – Eric Apr 15 '14 at 19:55

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