ArcGIS Desktop:Transformations: Geographic vs Projected?

Question

Arc10. I have two data sets (both CSV files w/ lat and long columns) and I am working on standardizing them to the same projection. File1: NAD27; File2: NAD27, WGS84, and NAD83. I would like all of the data to be in NAD83.

Step 1: I started my map by adding a layer that I know is in NAD83

Step 2: I added File1 and File2 (one at a time based on what the projection is). I attempted to transform all of the NAD27 and WGS84 data into NAD83, using Toolbox->data management tools->projections & transformations->project. I did not do anything to the NAD83 data points from File2 since they are already in NAD83.

Step 3: I then used data management->Features->add xy coordinates, so that I would have the new coordinates (based on NAD83) in the table and I can export to Excel and then import it to Access, produce queries, and map everything in the same projection.

The points in File1 have a 1:1 relationship with the points in File2. The problem is that I expected my points in File1 and File2 to map closer together once they were in the same projection. Instead, many of the points are further away from each other. I can't figure out what I did wrong. I wonder if this has to do with using geographic projections vs. using a projected coordinate system. I don't recall which I used, only that I searched for North American 83 and chose that.

Any ideas?

Answer 1

The use of geographic vs projected is more up to you and your analysis needs. Based on what I've read so far in this question, it is not necessary for you to project into a UTM zone. However if you aren't consistent in picking the correct type of CRS, you will get alignment issues. Generally speaking (and there are exceptions) if your coordinate units are in degrees, you need a geographic coordinate system. If they are in feet, meters, or anything other than degrees, you need a projected coordinate system.

Part of what may confuse you is that NAD83 is actually a datum, and you can see it as part of the names for both geographic and projected coordinate systems. In ArcGIS's dialog they are grouped into folders. When I search for "North American 83" I get no results, so I'm not sure what options you would have been presented with to choose from. When you first see the dialog there should be two folders - one for Geographic Coordinate Systems and one for Projected Coordinate Systems. If you expand the GCS folder, and then the North America folder under that, you'll see the NAD83 that you want. Note there are several variants as well, such as CORS, which are updates/revisions to the original NAD83 definition. But you can ignore these unless you know you need to use one.

I would go back to your original tabular data and redo the conversion from CSV to points with the Add XY Event tool. In that tool, make sure you select the correct GCS system for the coordinates. Then export those layers to feature classes/shapefiles. At that point each of your four files (one and the three parts of two) should have the correctly defined coordinate system.

If you then add them one at a time to ArcGIS, whichever you add first will become the dataframe projection. All the ones after that will automatically be reprojected on-the-fly to that first CRS. Note that because you're changing datums you'll need to select a transformation as mkennedy mentions. This is separate from picking a CRS, though done via the same initial dialog box (look for the transformations button), and if you don't select a transformation at all or choose the wrong one, that can also result in a misalignment. If things don't line up at this stage, there is an error in which CRS you selected - either because something you selected was wrong or because what they're labeled as in the CSV is wrong.

If everything looks like it lines up correctly, you can then use the Project tool on the non-NAD83 layers to convert them to that CRS. Alternatively, if your dataframe is in NAD83 you can right-click on them and choose export, and in the dialog switch from the source CRS to the current dataframe CRS option. When that's done you should have all four feature classes/shapefiles in NAD 83 and you could merge them or add your xy coordinates for whatever.

Answer 2

NAD1983 is a geographic coordinate system. There are a few different ways you could go about aligning these data, but I'll give an example that I think is simple and clean.

(1) Add both of your CSV tables to ArcMap and plot the points. Make sure you plot them using the correct geographic coordinate system - File1(NAD1927) and File2(NAD1983). Right click the CSV in table of contents > Display XY data > select coordinate system. Once they're plotted, export them as shapefiles (right click > export data).

(2) Use the Project tool to project both shapefiles into the same projection. This will give them the same map projection and datum (geographic coordinate system). Determine which UTM zone your data falls in and use that projection (e.g. NAD 1983 UTM Zone 14N). You may also choose to use a state projection system with the NAD 1983 datum. For the geographic transformation option, use NAD 1972 to NAD 1983 NADCON or one similar. You won't need this transformation for the dataset that is already NAD1983. Note: You don't have to project your data to a projected coordinate system, but there's no disadvantage to doing so. For cartography or spatial analyses you'll want projected data.

(3) Use add field (float or double) / calculate geometry to generate x and y coordinates for each of your datasets attribute tables so you will have coordinates for both in NAD1983.