Define projection changes position of raster layer

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

Ok i understand. But I have done as you said, I havent altered the data frame properties, I have only defined the projection for the layers that I have added to them. And since I get an error in each of the cases where I add  one of the two defined layers, I guess that both the defined projections are wrong.

I will make sure to make a description when/if I solve it.

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ChrisDonohue__GISP
MVP Alum

Let me see if I can help clarify part of this, as it is not obvious.  ArcMap takes the first data set added to it and inherits that data sets projection/coordinate system for the Data Frame in ArcMap.  For example, if I open a new MXD and then add a shapefile that is in "UTM 10N NAD 83" in ArcMap, the Data Frame will now be automatically set to " UTM 10N NAD83". 

When you are checking data to figure out what its coordinate system should be, you want to add it to a new ArcMap mxd that has not had any data added previously or had a projection/coordinate system defined.  This way you can get a real sense of the data's location.  If, however, one adds it to a MXD where it has already inherited or been assigned a projection/coordinate system, ArcMaps "project-on-the-fly" will be in operation, and the results will be not representative.

Chris Donohue, GISP

DitlevReventlow
Occasional Contributor II

Okay, in my case when I add the raster which I have defined the projection for, to a new arcmap file, I get the error message listed above, and as Dan said this means that the wrong projection has been attributed to the data.

So I don't know how to move from this. As mentioned, the original raster file had the following reference system User_defined equidistant_Cylindrical and the datum user_defined.

The only two I could think of that I could be would be equidistant_culindrical (Sphere) or equidistant cylindrical (world). And since both of them apparent are wrong (since I get the error message when I open them in a new mxd file).

I guess that the only thing I can know for sure is it is a projected coordinate system(because of the extent). But that is the only clue I have.

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MelitaKennedy
Esri Notable Contributor

Ditlev,

Can you post the complete, original definition? Please make sure it includes the spheroid (ellipsoid) information as well.

I think the projection parameters may be different. The "Sphere" and "World" definitions are generic and using zero for the central meridian and 60 for the standard parallel.

What happens if you just overlay this data (with its original coordinate system) with a base map or other reference map? Does it line up or not?

Melita

DitlevReventlow
Occasional Contributor II

Melita,

The original raster definition  is as follows:

"

User_Defined_Equidistant_Cylindrical

Authority: Custom

Projection: Plate_Carree

False_Easting: 0,0

False_Northing: 0,0

Central_Meridian: 0,0

Linear Unit: Meter (1,0)

Geographic Coordinate System: GCS_User_Defined

Angular Unit: Degree (0,0174532925199433)

Prime Meridian: Greenwich (0,0)

Datum: D_User_Defined

  Spheroid: User_Defined_Spheroid

    Semimajor Axis: 6378137,0

    Semiminor Axis: 6378137,0

    Inverse Flattening: 0,0

"

I create a mxd file with three data frames, one with the original raster and one with each of the newly defined raster. Then I add a basemap to each of these. This results in that for the original raster the dataset fits with the basemap. For the two where I have used define projection they don't align with the basemap, and they are very much off.

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MelitaKennedy
Esri Notable Contributor

Hi Ditlev,

So that was the issue. When you used the Define Projection, you changed the metadata, but not the actual coordinates of the raster, so it no longer reprojected correctly. You should try the Project Raster tool. Weirdly enough, the GCS/datum/spheroid/ellipsoid is a sphere with the same value as WGS84/GRS80 semimajor axis.

Hmmm. Actually, it may be easier to reproject the raster in ArcMap. Add it to ArcMap and set the data frame's coordinate system to what you want the new raster to use. Right-click the raster's name in the table of contents and select data, export data. In the tool, select the data frame's coordinate system. This will mean you won't have to create a custom geographic (datum) transformation to convert from the "GCS_User_Defined" to WGS 1984. I don't think it's needed.

Melita

DitlevReventlow
Occasional Contributor II

Thank you for this. I converted the coordinate system used by the dataframe to WGS_1984. This worked but it didn't shift any data around.

Then I went into data--> export data --> and changed the spatial reference from raster layer to dataframe and save. This worked in the sense that if I right click on the raster layer and select properties --> source, then the reference system states that it uses wgs_1984.

However, the file remains in the same position as the original raster. It didn't change around. Also If I look in data --> export data on the output raster, the spatial reference box still has raster dataset selected (and not data frame) although it is obvious that things have been changed around in the rest of the "export data menu".

What do you think about all of this? Do you think that it could be correct that the position of the cells are indeed the same in the original "defined" spatial reference system and WGS 1984?

MelitaKennedy
Esri Notable Contributor

What does the raster's properties say about its extents? If you add the new raster into ArcMap, ArcMap will reproject all layers to the data frame's coordinate system--so the two rasters should still line up. Is that what you're seeing?

DitlevReventlow
Occasional Contributor II

The two raster layers have complelety different visible extents. But they have the same full extent.

Yes the two raster are aligned when I open them in the same dataframe.

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DitlevReventlow
Occasional Contributor II

The original raster has the same visible and full extent (numbers around 15000000), while for the newly created, these are very different (the visible being much lower, like around-129).

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