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I need to measure the distance (with the ruler) of points from two different layers. However, one layer is NAD83 and the other is NAD27. I also have a standard ESRI basemap downloaded. The issue I have is that while ooints on the map all look fine, I don't know if they are messed up by the differing datum. I also don't know if I accurately converted from NAD27 to NAD83. When I uploaded my csv files an automatic message showed up, indicating that they would be converted to the NAD83. However, I admit to having no idea what the difference is between 'defining a projection' and 'projecting data.'

For Layer Properties -> Source , the following appears:

Geographic Coordinate System: GCS_North_American_1983 Datum: D_North_American_1983

  • Did you specify the correct datums when converting your coordinates on the csv files to xy point layers before the message came up about converting the datum? – Jezibelle Nov 10 '14 at 23:05
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    Hi Jezibelle, my apologies for getting so confused on this. I do not believe I did.... – WolverineTime Nov 12 '14 at 13:50
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    So: Define a projection - telling the software what projection and datum your data are in already. Projecting data: Changing the projection and/or datum to something other than the one currently defined – Jezibelle May 6 '15 at 5:24
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    Also, FYI, the differences in terms of location between the NAD27, NAD83, and WGS84 datums are fairly small (on the order of <100 m in most parts of the world) – Jezibelle May 6 '15 at 5:30
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You should specify the software you are using when asking this type of question. Based on other questions I gather you are using ArcGIS, which is capable of on-the-fly reprojection. This means it will automatically reproject data to the coordinate system (CRS) of the dataframe (which either you set or is set to that of the first layer you add with a defined CRS). If you add layers that use different datums, not just CRSs, you should get a warning about them not matching and needing to use a transformation.

When you create data from a coordinate list, you have to specify what CRS it is in (Define). If you have data in NAD27, you would enter that in the CRS box when using the XY Event Layer tools. NAD83 would be different. Adding both to ArcGIS would automatically reproject one to the other, with a transformation warning. I do not know to what you uploaded your CSV, or if using ArcGIS why you would see a message they were automatically being converted to NAD83. You might have seen they were being assumed to be NAD83.

Once you have your points in their correct CRS, you may want to convert one to the other permanently using the Project tool. See Layers with same coordinate system should align/overlap but do not? for a detailed explanation of the difference between Defining and Projecting. Note that on-the-fly reprojection, which looks similar in terms of requiring a transformation to be specified, does not actually modify the data.

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