Define projection changes position of raster layer

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07-05-2015 07:16 AM
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Occasional Contributor II

Hey Everyone.

I use define projection on a raster layer that has the spatial reference system User_defined equidistant_Cylindrical and the datum user_defined, and change it into either equidistant_culindrical (Sphere) or equidistant cylindrical (world).

I try to do it into both because I am not sure which of them is the correct one. My thought was then to convert into WGS_1984 afterwards and see which fits best.

However, when I do the define projection, the whole raster drastically changes position. How can this be? Isn't define projection just used to define the projection system, not actually changing the position or drawing. This is at least how it has normally worked for me.

The reason why I am doing this is because I want the data in WGS_1984. Any help on how I could do this is also needed.

I would very much appreciate any help. Thank you very much!

Best regards

Ditlev Reventlow

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21 Replies
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MVP Esteemed Contributor

Define Projection is only used when a file has an unknown projection or...as in your case...when it has been define incorrectly and has to be reverted back to the original projection

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Regular Contributor III

I didn't think define projection would move anything either but the project tool would if you had a projection assigned. Did you accidentally use the wrong one, maybe?

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Occasional Contributor II

As can be seen , it changes a lot but the two defined rasters are quite similar (not the same but quite similar)

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Occasional Contributor II

Dan, I am not sure what you mean by defined incorrecty. I mean I cant know whether it is defined incorrectly just because it is user defined?

Wes, As can be seen from the drawing both of them changes the position a lot.

Xander, the difference in my case is only about 10 km difference.

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MVP Esteemed Contributor

Your first sentence...you applied a projection to a data set to see which one was best or worked best.  Get the projection information from the source if it is known.  If it is a matter that only one of the two is correct, you will quickly find out...as long as you have ruled out the possibility that neither are correct.

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Occasional Contributor II

Dan,

Thank you for this. The thing is that both seems to be from. When I convert both the defined rasters to WGS 1984 with "project raster" , the almost dont change - they are still as heavily distorted. Aboth 10 km different as mentioned above, but both many 1000 km off other WGS 1984 rasters.

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MVP Esteemed Contributor

Is there a chance that you are doing these operations in the same data frame as other data?  I would suggest you create two new data frames and place each file into its own data frame with no other data.  Then ... for each file, in its own data frame...you right-click on the layer, go to properties and determine the extent values.  These will not change regardless of what you have done to the file in terms of defining a projection.  Do the extents look reasonable?  A file with a geographic coordinate system ( like a GCS WGS84) will only have values in the range -180 to 180 EW and -90 to 90 NS.  If they are big numbers then you have projected data and the range in values will give you what type of projection it was or should be in.  Ideally, both files should have the same extent if indeed they covered the same extent.  If they have the same extent and they are defined differently, then one or both have been improperly defined when using the Define Projection Tool.

Now on to projection... When you use the Project Tool, always put the result into its own dataframe since a file that is projected will project-on-the-fly to try and match the projection of the dataframe...and as you have noted, when the file was defined wrong, then projected, it flies off to somewhere you don't want it to be.

The whole process of adding data to a data frame since ArcMape is to try to "be helpful"... In the "old days" you would get the warning that things didn't match and sure enough, the files would fly off into their corners spatially separated because the projection files were wrong, undefined or mismatch.  In order to fix this in the "helpful" environment, you need to examine their properties...understand their possible extent values for a given coordinate system/projection and then act accordingly.

So in summary, I have no clue what the real coordinate systems of the input files were, but IF they were defined by some other source and they were redefined correctly...set them back and ensure that they are correct.  Once they are in a know, verified coordinate system...you can proceed to Projecting the data to a different coordinate system.  Then...and only then...do I introduce other data into the dataframe.  If all the files play nice...then they should overlap or abut perfectly.

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Occasional Contributor II

I get this error message I have attached to this message when I try to add either of the defined projections to a newly created data frame. I have shoen the extent of the two defined layers as well, As can be seen they are the same.

The extent values are far higher than they should be for GCS. So I guess that they must be projectied data. But this also fits with what they have been defined as since both World_equidistant_cylindrical and sphere_erquidistant_cylindrical are projected coordinate systems.

I am confused about what you mean when you say that the extent will not change when you define a new projection. Because at the same time you say that if the two layers were defined different (which they are ) and have the same extent then one or both have been improperly defined.

This to me sounds reverse.

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MVP Esteemed Contributor

I will try again...

You do not define the projection of the new data frame...you just add one file to it...without specifying anything.  If you get an error message...the file has been defined wrong.

If you define the projection of the data frame THEN add data...you are defeating the whole purpose of what I am trying to get you to do.  So I will defer to the guidance of others to try another approach to solve your problem.

At some time, everything will line up and you will have identified the problem...please report in detail what was done to rectify it so that others don't go down the path again.

Good luck

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