Plotting XY coordinates - DMS, DD, units problem?

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06-07-2018 01:06 PM
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New Contributor

I received some tabular data that I believe came from a GPS unit that had XY coordinates in a strange (at least to me) format - sample: 30.23.379, 097.44.220. I assumed these to be in degrees, minutes, seconds so I converted them to decimal degrees to plot in ArcMap. Plotting them (in NAD 1983) gets me in the right city, but they are not in the right location of the study area. See attached snip that shows the points and the study area that the points are supposed to fall within (in purple). The fact that the spread of points is 'larger' than the study area makes me think that there is some type of units issue.

I then converted the decimal degrees to feet and meters and tried to plot in State Plane and UTM, respectively, but no success - the points go to "no man's land". Likewise when I try to use the feet or meters coordinates and plot in NAD83.

Is there an issue with the original data? Or is there something else I can do to manipulate these points in the right location? Calling all you smart people!

FYI we have reached out to the contractor who supplied this data to inquire about the original units and coordinate system since that is unknown at this time.

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Esri Notable Contributor

I think those values are DD.MM.mmm AKA degrees decimal minutes rather than degrees minutes seconds. It's a very common format for GPS data.

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MVP Esteemed Contributor

Longitude is X and would be negative in the western hemisphere

Latitude is Y and would be positive.

Since they are in decimal degrees it needs to be defined (Define Projection in arctoolbox )as a Geographic coordinate system (GCS aka unprojected) with either a NAD83 or a WGS84 datum

If you need them in planar coordinates, then you need to use the Project tool in Arctoolbox

NAD83 is not a projection.... it is a datum

You need to know the projection you want  eg  UTM zone???, State Plane ??? Albers ??? with a NAD83 datum

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New Contributor

I should've added that I was sure to put longitude as X and latitude as Y. Also, have X as negative and Y as positive. My result got very close to the correct location, so if those parameters were off, I wouldn't be close.

I have plotted tabular data in ArcMap many times and always use either the NAD83 or WGS84 datum. I only started experimenting with feet/meters/projected coordinate systems because of this result. Ultimately we want this data to be in StatePlane with a NAD83 datum, but the points seem "scaled" improperly.

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MVP Esteemed Contributor

Well in the USA there is State Plane Feet and State Plane Meters, so if the difference is bit it is a meter/feet issue (3ish feet to 1 meter).  If the difference is much smaller (1-100 meters or so) then it is a datum issue.  if the difference in in the order of 100's of meters NAD27 versus NAD83/WGS84 datum

You can quantify the scaled issue for us?

Also... you didn't skip the Define Projection step did you?  It is needed prior to Project

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New Contributor

When I create the XY event layer, since it is temporary, I export the data as a permanent layer and it defines the projection as NAD83 (since the data frame was in that coordinate system). I would assume this to work the same as the Define Projection tool, or no? From there, I use the Project tool to define it as StatePlane, but the location does not shift.

The points are in the ballpark of 2-10 miles from where they should be. I don't know for sure since I don't the exact point locations.

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MVP Esteemed Contributor

NAD83 isn't a coordinate system... it is a datum

And you don't define it as the same as the dataframe.

Just use the Define Projection tool and specify it as either a GCS NAD83  or a  GCS WGS84 depending on what it actually is.  In either case the difference betweent he two geographic coordinate systems will be relatively small... not 20ft.

Once you define it properly, then you can project it

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I am plotting the points by right-clicking on the table in the TOC and clicking "Display X,Y". There I choose the GCS_North_American_1983 as the coordinate system of input coordinates and the event layer is created with points that are close, but not within the study area. Since this event layer is temporary, I export the layer as permanent and it keeps the GCS_North_American_1983 coordinate system as the defined projection. I just tested going to the Define Projection tool and it gives me the warning that a projection is already defined. From there, I have used the Project tool to get it in StatePlane, but this does not shift my points to the correct location.

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MVP Esteemed Contributor

Go to the little polygon study area file, right-click on it, go to properties and find the 'extent' and produce those numbers here.  One of the 2 files is wrong and it could be the feet/meters thing  Since State Plane comes in 2 flavors... one whose units are in feet and one in meters

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Esri Notable Contributor

I think those values are DD.MM.mmm AKA degrees decimal minutes rather than degrees minutes seconds. It's a very common format for GPS data.

View solution in original post

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MVP Esteemed Contributor

Melita, it still wouldn't account for the huge shift... do you have an idea as to what type State Plane might be used in that vicinity?

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