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WGS84 to NAD27 transformation   Heather Rudd Apr 18, 2008
Re: WGS84 to NAD27 transformation   Melita Kennedy Apr 19, 2008
Re: WGS84 to NAD27 transformation   Heather Rudd Apr 21, 2008
Re: WGS84 to NAD27 transformation   Melita Kennedy Apr 21, 2008
Re: WGS84 to NAD27 transformation   Heather Rudd Apr 22, 2008
Re: WGS84 to NAD27 transformation   Melita Kennedy Apr 24, 2008
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Subject WGS84 to NAD27 transformation 
Author Heather Rudd 
Date Apr 18, 2008 
Message Hello

I am trying to understand exactly how ArcMap displays data with localized datums. I want to map data in NAD27 Canal Zone. Generally if I have GPS data I bring it in as WGS84 and then project to NAD27 UTM Zone 17N using the _15 transformation (for Canal Zone). So far so good (I hope).

But since the data frame is in NAD27 UTM Zone 17 do the eastings and northings actually represent Canal Zone coordinates? Or do they shift to UTM 17N?

And what happens if the data is already in Canal Zone when it comes to me? If I bring it into ArcMap, I have to do it using UTM Zone 17N, not Canal Zone. I realize the difference is less than 10m, but for some applications this may be an issue.

I hope that makes sense. Any advice anyone can provide will be greatly appreaciated.

 
   
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Subject Re: WGS84 to NAD27 transformation 
Author Melita Kennedy 
Date Apr 19, 2008 
Message Hi,

If you get data in "Canal Zone", that means that it's already been converted to NAD27 using a transformation that specific for the Canal Zone. ArcGIS doesn't subdivide the geographic coordinate systems (aka datums) that way, so just make sure you use NAD27 (if lat/lon), or NAD27 UTM zone 17N.

Ah, I think this helps me understand your 2nd paragraph. Yes, because you transformed the data from WGS84 to NAD27 (using _15), the data is now on NAD27 geographic coordinate system.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: WGS84 to NAD27 transformation 
Author Heather Rudd 
Date Apr 21, 2008 
Message Hi there

Thanks for your quick reply! So just to confirm, if I use the Canal zone transformation (or have data given to me in Canal zone), and use NAD27 UTM Zone 17N to map in ArcGIS, I would be correct to put the reference coordinate system as NAD27 Canal Zone on the actual map (as opposed to NAD27 UTM Zone 17N)?

Thanks again

Heather 
   
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Subject Re: WGS84 to NAD27 transformation 
Author Melita Kennedy 
Date Apr 21, 2008 
Message You mean as metadata? That is, part of the map's text. If the map is NAD27 UTM zone 17N, you should use that.

Melita 
  Melita Kennedy
ESRI Product Specialist 
   
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Subject Re: WGS84 to NAD27 transformation 
Author Heather Rudd 
Date Apr 22, 2008 
Message Hi

Sorry, I guess I'm not being very clear. It's a diffcult question to articulate properly. And it may be there isn't really a good answer.

The issue I am having is this:

I am transforming GPS data to NAD27 using the Canal zone transformation. The shapefile/feature now reads as NAD27 UTM Zone 17N. Is it still Canal Zone?

The data is then mapped using NAD27 UTM Zone 17N in ArcMap. Other data in the map is in NAD27 UTM Zone 17N but the transformation is unknown.

So has the Canal Zone data shifted in the map or can it still be considered Canal Zone? Would the locations of the added GPS data be correct in relation to the rest of the map?

I hope that makes sense. I know the shift is small, but this issue has come up time and time again and I'd like to be able to address it once and for all.

Thanks again for your help

 
   
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Subject Re: WGS84 to NAD27 transformation 
Author Melita Kennedy 
Date Apr 24, 2008 
Message Hi Heather,

If by "Canal Zone" it means that the data was transformed to NAD27 using a transformation designed for the Canal Zone, then yes, it's Canal Zone. "Canal Zone" doesn't really have a meaning for the coordinate system. Do you have maps that say "Canal Zone" near the projection statement?

] So has the Canal Zone data shifted in the map
] or can it still be considered Canal Zone? Would
] the locations of the added GPS data be correct
] in relation to the rest of the map?

Or by "Canal Zone data" do you mean your existing data, where you don't know if it was transformed using the same transformation or created in some other fashion?

There's no telling unless you have have corresponding features whether the GPS data is transforming accurately or not. The transformation you're using is not known to be that accurate, but it's the only publicly known one available.

Melita 
  Melita Kennedy
ESRI Product Specialist