For ArcMap 10.3: I had two shapefiles, that at one point, were in different coordinate systems. Shp#1 was in NAD_1983_Texas_Centric_Mapping_System_Albers and Shp#2 was in NAD_1927_StatePlane_Texas_Central_FIPS_4203 and those were good times, as the shapefiles were then only 30 or so feet apart. But, I wanted complete accuracy in editing Shp#1 to Shp#2's minor detail change, so I attempted to change the coordinate system on Shp#2 in ArcCatalong by right clicking and going to properties and bring it over to NAD_1983_Texas_Centric_Mapping_System_Albers at which point it got even further away across my map in two completely different locations. I have performed a Define tool in ArcToolbox and a Project tool in ArcToolbox but to no avail-- it stays put in the exact same spot. I have checked MetaData-- no answers. The mouse hovers over the coordinates of each in meters as it should. I will probably geo-reference it as my last resort, but just out of curiosity I'd thought I'd try here to see if someone could explain to me why exactly something like this would happen.
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Changing the projection won't help if the projection is actually correct but the data is just off. Do you know which shapefile is more geographically accurate?– RHBFeb 17, 2016 at 21:06
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Yes, Shp#1 is accurate and aligns with imagery. Shp#2 was one I edited and created from a CAD drawing. According to Dan C. my defining it in ArcCatalog has somehow botched the process of an ArcToolbox Project, leading me to believe I will just have to redraw this .shp all together and begin with the correct coordinates. Having said that, this would NOT be a duplicate Q to the above post, as the above had a solution-- Project. Apparently there is no solution to getting a shapefile to project correctly after it has been wrongly defined in Catalog. That is what you're all stating?– NathanFeb 17, 2016 at 21:15
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It sounds like you just need to reset the original coordinate system (using the reverse of what you originally did), and then use Project (not Define Projection or Catalog to define it) to perform a projection (not coordinate system redefinition). That's all in the duplicate.– PolyGeo ♦Feb 17, 2016 at 21:51
2 Answers
Even if your shapefile were in different coordinate system, ArcGIS should project them on the fly so that they overlap properly. HOWEVER, ArcGIS does not always select the datum transformation: you have to go to the data frame properties > coordinate system > transformation and select one of the transformation (for most applications they are all suitable). My hypothesis is that your 30 m shift is due to datum difference but that your original shapefiles are correctly georeferenced (each in a different projection system). If selecting a transformation does not improve your accuracy, then I would add a third layer (e.g. a basemap) and try to find out which one is wrong.
In any case, do not use "define projection" to change the coordinate sytem of a layer that was provided to you except if you suspect a manipulation error from the data provider.
Project can be usefull in your case, but again you should make sure that you select the right transformation when projecting a data and that the "project" tool is applied on a dataset with the correct coordinate system. You can use the define tool to rollback your first modification before using the project.
Let's take an example : one of my neighbour is 1.8 m tall and the other is 5.5 feet tall. If I want to know who is the tallest, I need to convert their sizes.
I use an equation to convert from 5.5 feet to meters and I find out that 1.8 m > 1.68 m. This equation is like a projection.
Now if you don't trust the height unit, it is like defining that 5.5 is not in feet but in meter. In this case, one would be 1.8 m and the other one would be 5.5 m, so the second one would seem to be taller.
With project coordinate system there are several step for the conversion. 1) from XY to lat long on datum 1, 2) from lat-long on datum 1 to lat-long on datum 2 and 3) from lat-long on datum 2 to XY on target coordinate system. Step 2 (the transformation) is an approximation, but it is generally good enough in most cases. It is of course not necessary if the 2 projections use the same datum, but skipping it (like ArcGS often does) may result in a systematic error of 5 to 100 m (or a lot more if the prime meridian is different from Greenwich, but those cases are very rare).
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The Data Frame and the two .shp are all in NAD_1983_Texas_Centric_Mapping_System_Albers with GCS_North_American_1983– NathanFeb 17, 2016 at 20:12
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I am confused : you mention NAD 1927 in your post for the original shp number 2, but now you say that they are in NAD 1983– radouxjuFeb 17, 2016 at 20:19
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Yes. This was the original projection-- NAD_1927 fpr Shp#2. I then changed it's XY Coordinates through layer properties within catalog to Shp#1s coordinate system which is NAD_1983, at which point it got even further away. I then defined projection for Shp#2 to match Layer of Shp#1, which then turned Sp#2 into NAD_1983 but did not move the polygon, and it was still in the same spot. I then Projected it as a final attempt, to Shp#1s layer properties, NAD_1983... and it still did not budge. Did I do something wrong in this process?– NathanFeb 17, 2016 at 20:30
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changing projection in ArcCatalog is a way to define a new projection : you should not do this ! rollback shp2 to your NAD_1927_StatePlane_Texas_Central_FIPS_4203 (with define or with ArcCatalog), then modify the transformation for an accurate on the fly projection– radouxjuFeb 17, 2016 at 20:37
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Your layers do not have the same coordinate system. You mentioned:
so I attempted to change the coordinate system on Shp#2 in ArcCatalog by right clicking and going to properties and bring it over to NAD_1983_Texas_Centric_Mapping_System_Albers at which point it got even further away across my map in two completely different locations.
Selecting a different coordinate system that way does not change the coordinate system. You did the same thing the Define Projection tool does, which is to basically tell ArcGIS "This layer is in the specified coordinate system". It doesn't convert anything, and if you tell ArcGIS that your data is in one coordinate system when it's actually in another, things won't line up. If your data is in one coordinate system and you want to convert it to another, you need to use the Project tool instead.
Get Spatial's answer to this question goes into more detail: Layers with same coordinate system should align/overlap but do not?
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The answer you are eluding to and not stating in this comment about what I have not done, seems to point to use the Project tool in ArcToolbox, which I have already done. Unless I did not sift through the multitude of answers on the above post as thoroughly as I had time to, I assume that is what your are vaguely talking about? Am I correct?– NathanFeb 17, 2016 at 20:37
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Every time I come on this site, I never get a straight forward answer, just "see this post" or "you should NOT do this!" then I end up figuring it out for myself and it's usually a pretty basic fix. I'm beginning to question myself-- as in, "Why do I even come on here anymore?" There's a question.– NathanFeb 17, 2016 at 20:41
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@Nathan what it comes down to is that you should have used the "Project" tool. Right-clicking in ArcCatalog, clicking Properties, and changing it to a different coordinate system doesn't "bring it over" to that coordinate system.– Dan CFeb 17, 2016 at 20:45
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I used the project tool after the fact, as I have stated about three times. I'm looking for solutions, not past occurrences unless they have validity in the outcome. Do they?– NathanFeb 17, 2016 at 20:46
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@Nathan yes, using the project tool after doing what you did in ArcCatalog is part of your problem. Because you defined the coordinate system incorrectly in ArcCatalog, the Project tool is not working as it should. If you have the original files for SHP1 and SHP2, go back to those and use the Project tool on SHP2 to convert it.– Dan CFeb 17, 2016 at 20:48