topic Re: Plotting XY coordinates - DMS, DD, units problem? in Data Management Questions

Plotting XY coordinates - DMS, DD, units problem?

deleted-user-sXb0qqZ4LmSL — Thu, 07 Jun 2018 20:06:01 GMT

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.

Re: Plotting XY coordinates - DMS, DD, units problem?

DanPatterson_Retired — Thu, 07 Jun 2018 20:22:35 GMT

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

Re: Plotting XY coordinates - DMS, DD, units problem?

deleted-user-sXb0qqZ4LmSL — Thu, 07 Jun 2018 20:35:09 GMT

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.

Re: Plotting XY coordinates - DMS, DD, units problem?

DanPatterson_Retired — Thu, 07 Jun 2018 20:38:29 GMT

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

Re: Plotting XY coordinates - DMS, DD, units problem?

deleted-user-sXb0qqZ4LmSL — Thu, 07 Jun 2018 20:47:31 GMT

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.

Re: Plotting XY coordinates - DMS, DD, units problem?

DanPatterson_Retired — Thu, 07 Jun 2018 22:28:20 GMT

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

Re: Plotting XY coordinates - DMS, DD, units problem?

deleted-user-sXb0qqZ4LmSL — Fri, 08 Jun 2018 12:48:14 GMT

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.

Re: Plotting XY coordinates - DMS, DD, units problem?

DanPatterson_Retired — Fri, 08 Jun 2018 14:45:54 GMT

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

Re: Plotting XY coordinates - DMS, DD, units problem?

MelitaKennedy — Fri, 08 Jun 2018 22:31:32 GMT

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.

Re: Plotting XY coordinates - DMS, DD, units problem?

DanPatterson_Retired — Fri, 08 Jun 2018 22:46:36 GMT

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?

Re: Plotting XY coordinates - DMS, DD, units problem?

MelitaKennedy — Fri, 08 Jun 2018 23:41:55 GMT

It's Austin, Texas, so Texas Central zone.

NAD27 US survey feet, x = 2.5 million, y = 250k

NAD83 meters, x = 900k, y = 3 million

NAD83 US survey ft, x = 3 million, y = 10 million

NAD83 Texas statewide meters, x = 1.2 million, y = 900k

NAD83 UTM 14N meters, x = 600k, y = 3.3 million

There are 2 other statewide systems. It could also be in another state plane zone like south central.

Re: Plotting XY coordinates - DMS, DD, units problem?

DanPatterson_Retired — Fri, 08 Jun 2018 23:51:19 GMT

we will have to wait for Angela to provide the extents of the data then.

thanks Melita

Re: Plotting XY coordinates - DMS, DD, units problem?

deleted-user-sXb0qqZ4LmSL — Mon, 11 Jun 2018 13:21:06 GMT

Melita, it appears you are correct that the data is in degrees, decimal minutes rather than degrees, minutes seconds. Once I calculated decimal degrees with the correct formula, my points are now falling within my study area. This is exactly the reason that my data was "scaled" improperly. Thank you very much!

In regards to the projection/shifting issue, when I projected the data to StatePlane, I found that the data is actually (incorrectly) in the StatePlane Texas S Central zone, rather than the Central zone.

Thank you all for your replies!