WGS84 to NAD27

[DeanBixler]

I have some WGS84 coordinates in an excel table and I create a feature class from xy table in ArcCatalog.

I have to pass the data on in NAD27 but the project feature does not change the coordinate data in the output .dbf file.

Am I using the wrong tool for the job?

Thanks in advance for any help.

[MelitaKennedy]

Hello,

The viewable information in a DBF file is attribute information only. Coordinate values, etc are not updated automatically. If you have 10, you can use the Add XY coordinates tool (Data Management, Features), or in ArcMap, use calculate geometry for the fields.

Regards,
Melita

[DeanBixler]

Melita,

Thanks for your reply.   I ran the 'add xy coordinates' on my WGS84 data and then tried to project/transform to NAD27. 

The data is still identical although the properties properly reflect the projections for the 2 files.  How do I create a feature or simply a table with the corrected/transformed coordinates; in our case here there should be about a 200m shift in the northing/y and a 6m shift in the easting/x.

No GIS software is being used by my customer; a .dbf, spreadsheet, or CSV file is all they need/want.

Thanks again for your help.

Dean

[MelitaKennedy]

Hi Dean,

Did you project the data on-the-fly in ArcMap and then export? Did you set a geographic/datum transformation? The Project Tool will require a transformation if it's needed, but there are often multiple transformations between two geographic coordinate systems. NAD27/WGS84 has over 20 possibilities, mostly based on area of interest. What's the data extent?

If you used Project Tool, again, the coordinate in the attribute table (.dbf) are not updated automatically. The actual coordinate values are stored elsewhere if you're using a shapefile. You can update the fields in the associated dbf file as I mentioned before.

Melita

[DeanBixler]

Hi Melita,

What I have is some WGS84 data from a Trimble GPS receiver that is exported to an excel spreadsheet.

I use the 'create feature class from xy table' tool in ArcCatalog to create the shape files.  I then use the 'feature-project' tool under the projections and transformations section to create the NAD27 data; the transformation used is the nad27_to_wgs84_2.

I then run the 'add xy coordinate' tool on the nad27 shape file and the shift looks about right in the attribute table, 200+ meters in the y and around 6m in the x; which is what we expect in Belize.

I then open up an ArcMap project with the wgs84 data.  When I add the NAD27 layer the points are shifted by around 85 meters in the y axis. 

I have attached a 'print screen' showing the results.  I have a large project coming up after the first of the year and was hoping to use ArcMap to tie all of our data together; this conversion will be key in that operation.

Thanks again for your help.

Dean

[MelitaKennedy]

Hi Dean,

I can't tell from your post. Did you set the transformation in ArcMap as well?

Melita

[DeanBixler]

The projections and transformations were all done in ArcCatalog.  I read the WGS84 layer in first and the layer properties show that the projection is wgs84 zone 16n if that is what you are asking.

I can get the projected/transformed data to plot on top of each other by doing the following:

project wgs84 to nad83_1
project nad83 to nad27_nadcon.

But the 'add xy coordinates' to the final NAD27 file has different results than when going wgs84 to nad27 directly;  the difference seems to be close to that plot difference of 85 meters.

see attachment!

I am confused!

Dean

[MelitaKennedy]

Hi Dean,

Try setting the NAD_1927_To_WGS_1984_2 transformation in ArcMap. Open data frame properties, select the coordinate system tab, click the Transformations button on the right side.

Because you add the WGS84 data first, the "to" (pulldown) should list WGS84. In the top box, select NAD27 if it isn't. In the bottom pull-down select NAD_1927_To_WGS_1984_2. OK all dialogs. ArcMap has to be told which transformation to use, just like the gp tools.

The two step process you listed will give you different results. The first NAD83-to-WGS84-1 doesn't really do anything. It assumes NAD83 and WGS84 are equivalent. The second will only transform data in the lower 48 states (down to around 23N) so again no shift. The north-south offset is likely just due to convert the lat/lon values to UTM but with different spheroids (Clarke 1866 vs WGS84).

Melita

[DeanBixler]

Melita,

It looks like that worked; not exactly clear to the newbie as I thought all the stuff was done on the fly in ArcMap.

Let me play with my data a bit but  right now it looks good.

Thanks again for your help; you are a lifesaver.

Dean