Getting to the Point(s)

In this post we look at how points can be mapped from their raw coordinates in a text file or table.

So the GIS Helpdesk got a call last week – a postgrad was having some trouble mapping some points.  Ah, it reminds me of that time all those years ago, before I knew about GIS, when, after doing some GPS surveying I simply wanted to see all those points on a map so I could tell if they were in the right place.  I saw how easy it was and, for better or worse, I was hooked.

And that’s a familiar scenario – you’ve got some x and y coordinates recorded somewhere and want to translate them to a map.  It’s not a difficult task, once you know how.  So in this case, the student wanted to map the locations of meteorological stations in the Waipara catchment.  Amongst his 25 data files of rainfall, humidity and solar radiation, he had one that looked like this:

This is mapping gold and gives us all we need to get the points on a map.  Starting with the first line, we’ve got details about the station data; an ID number, name, latitude, longitude and elevation.  Then a line for each station with commas separating each item.  For mapping purposes it’s the latitude and longitude that we’re most interested in.  The negative in front of the latitude is pretty unambiguous – we are either north or south of the equator.  There’s a bit of ambiguity in the longitude as we could be either east or west, but the absence of a negative sign will be taken as east of the prime meridian at Greenwich.  Importantly, these are in decimal degrees rather than degrees minutes and seconds, which makes things a tad bit easier.  So a quick inspection shows us that we’ve got the data we need to map these eight points.

Next, we’ll add this text file straight into ArcGIS – I’ll use Pro here and just add the text file straight onto a new map:

We could just have easily added an Excel sheet or a dbf table.  Once on the map I can open the table with a right-click.  Because it uses commas to separate values, it’s easily read in:

To map those points, all I need to do is right-click on the table name and go to “Display X Y Data”

This will open up a new tool in a pane at the right where I can set some parameters about which field is my x coordinate and which my y.  It’s “smart” enough to recognise Long and Lat appropriately:

It’s also recognised that the data’s coordinate system is WGS84 (the lat/long system used with GPS).  Clicking “Run at the bottom of the tool should deliver up some points on the map:

Et voila!  I’ve also added the Waipara catchment boundary to the map to give it some context.  If I’m happy enough with this for mapping purposes, I could next create a layout.  If I were thinking of using these points for some analysis, my next step would likely be to convert the layer’s coordinate system from lat/long to New Zealand Transverse Mercator, as the rest of my data would likely be in that system.

So mapping points is quite easy as long as you’ve got the right information (the coordinates).  And that’s the whole point.

C