Display XY Data not honoring coordinate system of input coordinate system?

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02-10-2015 12:34 PM
SteveRobischon
New Contributor III

Display XY Data not honoring coordinate system of input coordinate system - I have tabular data with lat lon columns in an unknown geographic coordinate system I need to bring into ArcMap and compare to a feature class with a known coordinate system.  I used Display XY Data three different times, each time specifying a different geographic coordinate system (NAD27, NAD83, and WGS84) for the input data.  I exported each to a new feature class and displayed them all on a map.  I expected there would be a difference in the point locations between the NAD27 and the NAD83 points, but all 3 essentially fell on top of each other.  Is there something I don't understand about the Display XY Data functionality, or is it not honoring the specification of input data coordinate systems, or what??

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MelitaKennedy
Esri Notable Contributor

ArcMap doesn't automatically set geographic/datum transformations any more. It used to automatically load NAD_1927_To_NAD_1983_NADCON but we stopped doing that a few years ago (10.1?). If you look at each shapefile individually, yes, they'll all have the same coordinates. If you add two or all of them into ArcMap and don't set any transformations, they'll still have the same coordinates.

Transformations are set on the Coordinate Systems tab of data frame properties.

If the data's in the US, try NAD_1927_To_NAD_1983_NADCON for NAD 1927 and NAD 1983. You can use NAD_1927_To_WGS_1984_79_CONUS between NAD 1927 and WGS 1984 if you want to use the same conus grids from NADCON. For NAD 1983 and WGS 1984, you might try WGS_1984_(ITRF00)_To_NAD_1983.

Melita

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10 Replies
DanPatterson_Retired
MVP Emeritus

The coordinate system of the data frame is established by the first file added to ArcMap unless otherwise changed.  After that, any file that is added with a different coordinate system is 'projected-on-the-fly' to that of the data frame so that everything automagically appears to align giving people the ability to view data with different systems in one common value.  That is great if you know that this is happening, but a disaster if you don't or you want to view data in 2 different coordinate systems...and in your case...datums...to see what the differences might be.  You have been bitten by the Esri-helpful-bug and it appears that you don't want to see the data the way they want you to see it, but you would rather see it for what it is.

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SteveRobischon
New Contributor III

Dan,

I understand what you are saying about on-the-fly projections, but I'm not sure I agree with  your conclusion.  Consider the process I followed:

I started with an Excel spreadsheet containing columns for Latitude and Longitude in an unknown coordinate system

I then opened a blank map (so no coordinate system defined for the Data Frame) and added the sheet as a table.

I then did Display XY Data, selecting NAD27 as the input coordinate system for the tabular data

I then exported the events layer to a feature class named NAD27.shp

Next, I closed and reopened Arcmap, and started over with a new blank map (so again no Data frame coordinate system)

I repeated the above process, this time choosing NAD83 as the input coordinate system, and exporting the result to NAD83.shp

I repeated the process again, choosing WGS84 and exporting to WGS84.shp

So at this point, all 3 feature classes have been created independently from the same tabular data.  But since i chose a different input coordinate system for each, and each coordinate system references a different ellipsoid, the physical location of latitude-longitude coordinate pairs should fall on a different spot.  I used the same (literal numbers) coordinate pairs to create all three feature classes, so they should fall on different spots.  40N117E in NAD27 is not the same location as 40N117E in NAD83 - in my part of the country the offset is somewhere between 80 and 240 meters.  By importing the tabular data "independently" in all three cases, the result of the process should have "frozen" the locations of the points at different locations.  The on-the-fly reprojection in ArcMap would then merely project all three feature classes to a common coordinate system, but still at different locations.

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TomSellsted
MVP Regular Contributor

Greetings Steve,

When you are adding data from a spreadsheet and using the Display XY Data, you must specify the projection of that dataset for it to be properly projected onto your existing map.  So if your map is in a state plane projection and your spreadsheet coordinates are actual latitudes and longitudes (WGS84), set the coordinate system when displaying them to WGS84 and then they can be projected on the fly to match your map coordinates.  ArcMap has no other way to automatically interpret the coordinates in your spreadsheet.

Regards,

Tom

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SteveRobischon
New Contributor III

Hello Tom,

I did specify the coordinate system of the input data in the Display XY data window.  The events layer was then created in ArcMap, and as I had not previously set the coordinate system for the data frame, it uses the coordinate system I specified for the input data - the first spatial layer added to the map (e.g. begin with blank map, no data frame coordinate system defined, Add XY Data, specify input data coordinate system (as either NAD27, NAD83, WGS84).  Export resulting events layer to feature class)

Regards,

Steve R

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TomSellsted
MVP Regular Contributor

Steve,

Apologies, I may have misunderstood you.  I had thought that you were relying on ArcMap to interpret the projection of data that was unknown. 

Regards,

Tom

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MelitaKennedy
Esri Notable Contributor

ArcMap doesn't automatically set geographic/datum transformations any more. It used to automatically load NAD_1927_To_NAD_1983_NADCON but we stopped doing that a few years ago (10.1?). If you look at each shapefile individually, yes, they'll all have the same coordinates. If you add two or all of them into ArcMap and don't set any transformations, they'll still have the same coordinates.

Transformations are set on the Coordinate Systems tab of data frame properties.

If the data's in the US, try NAD_1927_To_NAD_1983_NADCON for NAD 1927 and NAD 1983. You can use NAD_1927_To_WGS_1984_79_CONUS between NAD 1927 and WGS 1984 if you want to use the same conus grids from NADCON. For NAD 1983 and WGS 1984, you might try WGS_1984_(ITRF00)_To_NAD_1983.

Melita

SteveRobischon
New Contributor III

Melita,

Based upon your reply, I messed with the data a little, and now I understand what's happening.  So I guess the bottom line is that, even though you think you're getting on-the-fly projections with ArcMap, when you're working with data with different horizontal datums, unless you explicitly set a transformation, you'll end up in trouble.

I remember the graphic the shows projections as proceeding from "flat" back to "ellipse", then across to "other ellipse", then down to "other flat".  So I guess what's now happening is, unless you tell ArcMap to go back to the "ellipse" and across, it just jumps across the "flat"??

Thanks for the help.

Steve R

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MelitaKennedy
Esri Notable Contributor

Steve,

That's right! If part of the coordinate system conversion is from geographic coordinate systems A to B, if no transformation has been set, the software essentially redefines the data as being on GCS B, and goes on its merry way.

Melita

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JanettSaid
New Contributor

I am encountering the same issue. My new points from XY information are not aligning with my boundary shape files. I checked and both Project Coordinate System and Geographic Coordinate System are exactly the same, also the same for the Data Frame. Any suggestions? MKennedy-esristaff

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