Ask A Cartographer

Plotting of GPS points comes up wrong

July 05 2007 | 0 comments
Categories: Map Data

I just learn how to use ArcGIS and I have a couple of questions. I am working with marine mammal data at sea and I am trying to plot GPS points of sightings on a map of Santa Monica Bay, CA. I worked on a chart I found on the web and I was able to plot the GPS points. After a few months, I tried again but the points continue to come up wrong. I am attaching the jpeg
of the incorrect chart.

These two shapefiles are almost identical (one has a few extra records) and when given the geographic projection NAD 1983, they do not appear the same in ArcMap. The incorrect shapefile appears square, whereas the correct shapefile follows a concave contour on the right side. Both shapefiles were created using the same Access database table, displaying the X,Y data to create the event layer and then exporting that event layer to create the shapefile. What could be the problem? Why do they appear differently? How may this problem be solved?

I would also like to know if you have good quality nautical charts of this area that I can use to plot my GPS points for the sightings. I need a good coverage for Southern California Bight and a more detailed map of Santa Monica Bay. I need to use these maps for scientific publications so they need to be clear and with the bathymetry.

Please, let me know.

Thank you for your help.


Attachment: Incorrect chart of study area

Mapping Center Answer:

First, I would make sure the coordinates in the Access data are what you expect. I sent back a copy of your JPG file, but added two red circles; find those two points in your access data and make sure they have the same or relatively speaking, very similar Y coordinates.

When I’ve brought GPS points into ArcMap, I used the XY Event dialog box (tools menu) and as a software function it is really simple (plot points), but heavily dependent on your entering the correct coordinate system. More than once I’ve failed to do this consistently and got similar mismatches my on-screen data. The trick I use is first make sure I’ve got the coordinate system that’s needed and choose it for the first dataset. Then I export that data to a shapefile. For any subsequent GPS datasets, I import rather than select a coordinate system, importing from the shapefile I first created—this guarantees I’m not accidentally selecting a different coordinate system.

As for hydro charts and bathymetry data, I’m no expert. I would look to NOAA, and maybe even email one of their staff for a recommendation.

If you would like to post a comment, please login.

Contact Us | Legal | Privacy |