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I asked an earlier question about Geocoding (I initially called it Georeferencing, but have since learned the difference) addresses from the Saint Louis Metropolitan Police Department (SLMPD) crime data. I wound up having to geocode using ArcGIS online to project to a basemap, and then downloaded the resulting projected layers to desktop (ArcMap). That part is fine.

The part that is NOT working is that when I try to add these layers to my other layers (projected at NAD_1927_StatePlane_Missouri_East_FIPS_2401), by projecting them from their source data (WGS_1984_Web_Mercator_Auxiliary_Sphere) to _1927, it doesn't work. It consistently projects it to the exact same location it has as _1984, which is slightly higher and to the left of _1927. I know this can work. I did it with an earlier section of data from SLMPD via the ArcGIS online method. The problem is I did not keep a record of how I fixed it, nor do I remember how I did it. I have tried projection to _1927, projection to _1983(US feet) and THEN projection to _1927, batch projection in the same manner, creating a custom GeoTransformation from _1984 to _1927 and then Projecting, and by defining projection to another source and then projecting to _1927. None of them have worked. I think I'm stuck because I'm fixating too much and can't see outside the box, so I thought I'd ask for suggestions from the GIS crowd.

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  • Are you sure your data is actually in NAD27, and not NAD83 (which is very close to WGS84) ?
    – Dan C
    May 10, 2017 at 16:26
  • Yes. I have specific data layers (ex, Saint Louis city streets) projected to both _1927 and _1983, and use the applicable projection for different maps that I work on. I am using the _1927 one right now. Properties > Source confirms it is _1927.
    – T. Kerr
    May 10, 2017 at 16:50
  • So here is what I currently have: Tried Projecting March2017_MergeB to _1983 via Geographic Transformation “NAD_1983_To_WGS_1984_1” to create March2017_MergeB_1983B The result is VERY close to the correct data points, just off to the right by a hair. Next try Projecting _1983B to _1927 using GeoTransformation: “NAD_1927_To_NAD_1983_NADCON” to create March2017_MergeB_1983_1927B Overlaps _1983B exactly. This is still off by a hair - about two centimeters to the north of where it should be when I zoom in to look at the parcels (comparing data point with address on googlemaps).
    – T. Kerr
    May 10, 2017 at 16:53
  • Geographic transformations aren't perfect, especially when you're transforming from NAD27, which was not very accurate in some parts of the US. 2cm seems like it's well within NAD27's margin of error (which was multiple meters in some areas if I recall correctly).
    – Dan C
    May 10, 2017 at 17:00
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    In ArcMap, open data frame properties, coordinate system tab, and click the Transformations button. Make sure WGS 1984 is selected in the top box. In the bottom pull-down, select NAD_1927_To_WGS_1984_79_CONUS. This uses the same CONUS file as “NAD_1927_To_NAD_1983_NADCON”. Two cm is nothing. The accuracy of this transformation is worse than that.
    – mkennedy
    May 10, 2017 at 17:59

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So I think I may have fixed it. I opened a blank map, projected everything to 1927 and then reprojected everything to _1983. The points are still on the street instead of on the parcels, but otherwise correct (point A for example is directly in front of its relevant parcel address), and ALL of them projected on the street like this.

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