How to identify the data not actually matching the spatial reference?

Question

Hi Everybody:

I am making a data manipulation tool to handle some WGS84 data.
    There is some data with "fake" WGS84 spatial reference, which means: when I use ArcCatalog to look at the property of the datasets and feature classes, it shows WGS84. However, the underlying data is actually NAD83. The tool I developed will not give correct result with this kind of data. How can I identify these kind of data? I want to put some data validation logic to the tool to reject this data. Any idea>
    Thank you!

SuiHuang · Answer

Hi Melita:

Here is the way we know that there are some "fake" WGS84 data:
1. we have some new data that we are sure they are in WGS84
2. we have some old data labeled with WGS84 in property
3. we display both new data and old data in ArcMap together, and find that they are not fitting each other well (there is offset), then we know the old data is actually in other GCS
4. we manually modify the old data property to NAD83, and added a NAD83-->WGS84 transformer in MXD.
5. the display of new and old data fites together now.

I don't think the data is projected. Here is the GCS information I observed in layer properties:
************************************************
Geographic Coordinate System: GCS_WGS_1984
Datum:  D_WGS_1984
Prime Meridian:  Greenwich
Angular Unit:  Degree
************************************************

Any further idea? Thank you!How do you know that the underlying data is NAD83? Is the data really in a projected coordinate system based on NAD83? If that's the case, you could check the data extents. Data in a projected coordinate system will have much large extent values than data in a geographic coordinate system.

Melita

MelitaKennedy · Answer

Hi Everybody:

I am making a data manipulation tool to handle some WGS84 data.
    There is some data with "fake" WGS84 spatial reference, which means: when I use ArcCatalog to look at the property of the datasets and feature classes, it shows WGS84. However, the underlying data is actually NAD83. The tool I developed will not give correct result with this kind of data. How can I identify these kind of data? I want to put some data validation logic to the tool to reject this data. Any idea>
    Thank you!

How do you know that the underlying data is NAD83? Is the data really in a projected coordinate system based on NAD83? If that's the case, you could check the data extents. Data in a projected coordinate system will have much large extent values than data in a geographic coordinate system.

Melita