NAD27 to NAD83

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08-30-2011 05:25 PM
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by Anonymous User
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Original User: CJHatTRWD

Hi all-

I have a few questions to ask about a project I am working on. I have scanned maps from the 70's into .jp2s, and am having a hard time converting the coordinates from NAD_1927_StatePlane_Texas_North_Central_FIPS_4202 to NAD_1983_StatePlane_Texas_North_Central_FIPS_4202. The coordinates given on the maps are in NAD27, so I used the Georeferencing toolbar and control points to place the points on the image in a workspace defined in NAD83 (images at this stage are NAD27 because of the coordinates on the map). So, when the maps are in NAD27, they are in the correct place in my workspace, but, when I right click a layer and go Data -> Export Data to change the projection, or use Define Projection, or Project Raster, ArcMap takes my NAD27 image and throws it way south of where it needs to be. So, I have two main questions: 1) Why is my image being placed south of where I need it to be and 2) when I use Data ->Export Data (which is what I've had the most luck working... sometimes) the image that is exported is nowhere near the quality of the original, and there are little black dots all around the image that end up looking somewhat like a blackline border. Anyone know anything about how to fix?? Thanks!
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Esri Notable Contributor
Hi,

in NAD27, Texas North Central uses US survey feet and has:

false easting: 2000000.0 USft
false northing: 0.0 USft

In NAD83, it can use meters or US survey feet. The meter version uses:

false easting: 600000.0 m = 19685000.0 USft
false northing: 2000000.0 m = 6561666.66667 USft

I am wondering if there's some confusion in the georeferencing. Once you georeference the rasters, is the coordinate system listed as NAD27- or NAD83-based? And is that information correct? If the coordinate system is set to NAD83-based, but you georeferenced with the NAD27-based coords, that would case the raster to show up south of where it should be.

I think you should georeference in a NAD27-based workspace (set ArcMap to use NAD27 TX NC), define the coordinate system, then use the Project Tool, or set the data frame to NAD83 TX NC and use data, export data.

Hope this helps,
Melita
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