# Datum transformation

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05-24-2016 03:53 PM
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New Contributor III

I am doing exercise "Troubleshoot Datum shift" in certificate "Working with Coordinate Systems in ArcGIS"

I converted the datum to have the point hydrants in the parcels (as per instructions):

Then the problem occurs because some of the hydrants are just a bit outside of the parcels, and not inside.

As per instructions, I change from NAD27 to NAD83 under "Coordinate Systems" and I get the following message:

After I click Yes, all the hydrants are inside the parcels.

However, I experimented for my own interest: I didn't do the data transformation but skipped to the step of changing from NAD27 to NAD83. The results is the same. Thus, what is the purpose of going through the data transformation?

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MVP Esteemed Contributor

a Nad27 to nad83 transformation when projecting, will move the points to the east and north in my region (UTM zone 18N), which is a good thing otherwise your data will be offset.  It is always a wise idea to project your data to a common coordinate system (like a UTM xx NAD83) and use the transformation while you are doing it.  The reason for the datum shift is because our knowledge of the shape of earth changed due to advances in technology.  When the 'shape' of earth changes, so will coordinates if we keep the references points (Greenwich and the equator) fixed.  Think of blowing up a circular balloon, as it gets bigger, there are still 360 degrees around the equator (and all other points) but the circumference (hence the equatorial and polar radii) changes.

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Esri Notable Contributor

How do they have you changing the data from NAD27 to NAD83? Do you use the Project Tool or the Define Projection Tool or ArcCatalog, data's property page?

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New Contributor III

The instructions are to change it under the Data Frame properties, under the Coordinate System tab.

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Esri Notable Contributor

I think I understand now. Even though you changed the data frame to NAD 1983, you still need to specify a transformation from NAD 1927 to NAD 1983. ArcMap won't pick a transformation automatically. Any WGS 1984 layer (data) should be around 1 meter or less away from NAD 1983 data so setting a transformation for that pair is sometimes not necessary.

Open up data frame properties, coordinate systems tab, Transformations dialog again. If the "Into" GCS is NAD 1983, select the GCS_North_American_1927 entry in the top box and make sure there's a transformation set.

I hope this is useful.

Melita