Feature Projection error

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08-21-2017 04:01 PM
FaithWebster
Occasional Contributor

I have 2 shapefiles from a few years ago that I did not author. One is in NAD27 UTM 10, the other is in NAD83 UTM 10. . There was a map request that includes this data. We have a standard practice of using NAD83 HARN SPS. The Data frame and all other associated data is in SPS. When I added the layer the data was off by a few meters, and I assumed it was a projection issue. I projected the UTM data into SPS, but the points did not shift into their correct locations, they are about 35 m off according to the measure tool. What can I do to fix this? Thank you  

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5 Replies
MicahBabinski
Occasional Contributor III

Hello Faith,

Did you apply a geographic transformation, either in your data frame when you set it to NAD83 State Plan or when you projected your shapefile to NAD83 state plane? Because your data sources use different datums, represented by the different geographic coordinate systems (remember, a projected coordinate system like State Plane Oregon North or UTM Zone 10 is comprised of a projection and a geographic coordinate system, among other parameters), you'll need to use a geographic transformation to project the data.

For your purposes I would recommend this one:

Let us know if that works for you!

Warm Regards,

Micah

P.S. Here is some good information about geographic transformations:

About geographic transformations and how to choose the right one | ArcGIS Blog 

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FaithWebster
Occasional Contributor

Thank you for the refresher on transformations and projections. I consider myself generally well informed, but I have been known to miss the obvious in the past. 

I did use transformations when I  I projected the NAD 83 UTM, but on your suggestion I double checked what was used so I could report back accurately. I selected the NAD 83 to NAD 83 HARN_WA_OR, or something on those lines, but when I check the results the NAD_1927 To_NAD_1983_NADCON transformation was used instead. I tried it again, and the same result. I really have no idea what is going on there or why.. or how...  On your suggestion, I projected the 27 data from the NAD 27 UTM set to SPS with your recommended transformation. The points are still not where they need to be. The 27 data did move with the projection. but not to the correct spot. 

If you have any other thoughts, I am all ears. 

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MicahBabinski
Occasional Contributor III

Hmmm, is it possible that one of the shapefiles has its coordinate system defined incorrectly? Sometimes it helps me to consider the source of the data - was it GPS? Digitized based on aerial photo or other reference data?

If I think that my data source has the wrong coordinate system definition, here is what I do:

  1. Open a blank MXD
  2. Make a copy of the dataset that is not lining up correctly in ArcCatalog
  3. On the copy, set the coordinate system to Undefined in ArcCatalog
  4. Set the data frame of the blank MXD to the geographic coordinate system you think the mystery data source might have
  5. Add a dataset that you know has the right spatial reference
  6. Add the mystery dataset

Repeat these steps (besides step 2) until it lines up correctly. The geographic coordinate system which you set your data frame to is the right one. This has worked for me in the past, but if you post your shapefiles here maybe some folks who are better at it than me could take a look.

Good luck!

Micah

FaithWebster
Occasional Contributor

I will try that! unfortunately, I cannot post the files. They are proprietary, and highly confidential. But they were manually digitized from a descriptive document. The projection could have been defined incorrectly. I don't know the skill level of the person who made it, other than they have done some very odd things. 

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MelitaKennedy
Esri Notable Contributor

If you're checking the transformation in ArcMap, make sure you've selected the correct "from"/source GCS in the top box. That will change the list in the Using pull-down and show what's set for each unique from/to pair of GCS.

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