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I've been working on a bunch of layers of differing co-ordinates projections within a single dataframe. Because I haven't really got my head around this, and because its all been working just fine, I didn't want to touch it.

However, one of the functions I was trying to perform wouldn't work (extract multi values to points) and I suspected this was because of coordinate conflicts.

My data frame properties show that I have layers with the following coordinates in it:

  • GCS_WGS_1984

  • NZGD_2000_New_Zealand_Transverse_Mercator

  • NZGD_2000_Transvers_Mercator

  • NZTM

I guess they should all be on the same.

I have just tried to change the projection of 3 layers (why didn't I check after the first one???) from GCS_WGS_1984 to NZGD_2000_New_Zealand_Transverse_mercator using "Define projection tool". They've gone missing, even zooming to layer wont find it.

The properties of the missing layers show the extent in lat/long. The properties of the layers I can see (haven't changed the projections of) show the extent in meters.

I'm wondering if this could be the issue, and if it is how to change it. If its not this, what could it be? Whats the best way forward?

I have re projected (as per advice) and can now see my layers on the same page - but they still don't overlap. I suspect this is because of extents. The attached images will show where the layers of extent type A and B respectively are showing. Projecting has not changed this... is there a way to change it? Maybe try another co-ordinate system?

Locations where layers are showing. Layers in A and B have the same coordinate system (NZGD_2000_New_Zealand_Transverse_mercator) but the extents are different...

Extent for layers in B - shows recognisable lat/long (175, 34[![][2]]4

Extent for layers in A - shows no recognisable coordinates

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  • GCS_WGS_1984 is not the same as the other three (NZTM). Why are you thinking you need to change them? As per @Radar's answer below, to change them you possibly need to Project rather than Define Projection, although three of them are already the same projection.
    – Midavalo ♦
    Commented Oct 7, 2016 at 22:53
  • I thought some were the same, bu the fact they weren't showing made me think I had to change them so they would all be the same. I'm relatively new to this, and so that would have ruled projections out. It didn't, but I think that might be because the extents are set differently - lat/long on some layers, meters on others.
    – Malanus
    Commented Oct 7, 2016 at 23:15
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    If you "clear" the data frame's coordinate system (properties, coord sys tab, globe icon) you should be able to see 2 groups: the lat-lon layers and the NZTM ones. The lat-lon ones need to be assigned WGS 1984 again. If there are more groups, it means that there's more than one projected coordinate system involved.
    – mkennedy
    Commented Oct 7, 2016 at 23:53
  • I've just tried this, but I'd already re-projected the co-ords so all layers have the same - NZGD_200_New_Zealand_Transverse_Mercator (maptoaster.com/maptoaster-topo-nz/articles/nztm/nztm.html). Not sure how they can have the same projection but different extents though. Screenshots show where extent A & B show respectively - they should overlap. "B" has the right co-ords for the area. They did overlap before i tried to re project - but I didnt note what I was changing them from to change back :(
    – Malanus
    Commented Oct 8, 2016 at 0:29
  • (it is possible that I'm being completely dense...)
    – Malanus
    Commented Oct 8, 2016 at 0:34

1 Answer 1

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The Define Projection tool does not change the projection (i.e. it does not alter your data), it only overwrites the coordinate system information in the source view. Using this tool is like sticking a Ford insignia on a Dodge truck - it doesn't change the fact that it's a Dodge truck, it'll just confuse people at first glance.

If you want to change the projection from one coordinate system to another you will want to use the Project tool.

Use the Project tool to put all of your data in a single coordinate system and you'll be able to use other geoprocessing tools.

The most commonly used projection in your case would be GCS_WGS_1984, but what you choose will be application and location specific.

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    Thanks Radar. I know less about cars than GIS, but your analogy made perfect sense ;) It's like taking a GIS paper at uni, and everyone thinks you know something about it but behind the scenes, you're faking it ;) I used the "batch project" tool because there are quite a few layers. On re adding them, I have had a warning about the extents being inconsistent so it looks like it hasn't changed the fact that some are in meters and others in lat/long... I will try figure out how to fix this (i see others have had the same issue, and see if it works)
    – Malanus
    Commented Oct 7, 2016 at 23:10
  • Right - its all fixed.. The problem was well explained by Radar (see post above). I had used "define projection" which was the wrong tool. To fix, I used "define projection" tool again, reverting the layers back to their initial coordinate system. Then I used the "project" tool. Thanks for your help :)
    – Malanus
    Commented Oct 8, 2016 at 3:17

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