Ask A Cartographer

Imported Data and Maps are Not Aligned Correctly

April 14 2010 | 0 comments
Categories: ArcGIS Methods, Map Data

Note: I am a new user. I have created a map (data frame?) which incorporates US Topo Maps and World Imagery, and a layer of State lease boundaries. I have also created another layer of geothermal well locations throughout the State of CA. The US Topo maps and World Imagery were downloaded from ESRI and are in GCS_WGS_1984 format. The well locations are plotted as lat/long in NAD27. The well database contains hundreds of wells statewide, whose locations have been plotted individually in a variety of methods over a period of over 50 years, and imported as a shapefile from another State agency. When I recently downloaded the World Imagery map, I noticed that the plotted well locations were several hundred feet off their wellpads on the World Imagery map. When I searched Statewide, I noticed that most if not all well locations were plotting with about the same misalignment, or about 300 feet east of their respective wellpads. In some locations, the actual wells are visible in World Imagery, but their plotted locations are well to the east. In the attachment you can actually see three wells (as gray points) on a wellpad with the 3 plotted locations to the east, in the same configuration. Where might the error lie and how can I correct for it Statewide without having to re-plot each individual well?

Mapping Center Answer:

It sounds like you don't have any datum transformation set between NAD27 and WGS84. We automatically set one for NAD27 / NAD83, but not NAD27 / WGS84.

Here is what you should try:

1. Open Data Frame Properties and select the Coordinate Systems tab.

2. Click the Transformations button (middle of the right side).

3. The pull down in the middle, labeled "Into:", lists the data frame’s coordinate system. Let's assume it's WGS 1984. The top box will list all unique geographic coordinate systems used by the map and layers. Select NAD 1927 (GCS_North_American_1927).

4. In the "Using:" pulldown, select NAD_1927_To_WGS_1984_79_CONUS.

5. Click OK to all dialogs.

Note: If the map's coordinate system is NAD 1927, select WGS 1984 in the top box instead and then do steps 4 and 5. Don't worry about the directionality of the transformation. The software will apply it correctly.

There are also some previous Mapping Center blogs written on this topic that you may find useful:

GCS WGS1984:  Why should you care about it?

About geographic transformations and how to choose the right one

Specifying multiple transformations for your data frame's coordinate system




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