Fishing & Cruising Charts for Lough Corrib

& lakes in the West of Ireland

All GPS's are NOT the same

Garmin and Lowrance tracks 107ft apart

This image shows the tracklogs, with colour coded depth data, taken simultaneously on a Garmin GPSMap 60csx and a Lowrance HDS 8 unit. Both systems had exactly the same uninterrupted view of the sky. At 1425 the Garmin GPS started to wander, and a few minutes later was roughly 107ft from the position indicated by the Lowrance unit. The Lowrance unit was indicating the correct position to within a few metres throughout, verified by both local knowlege and depth readings as I passed over a previously charted shallow patch.


Had I been using only the Garmin unit and not navigating by eye as well, and been unfamiliar with the lake I would have been in serious trouble. A compensation to starboard to keep the Garmin track between the rocks and the headland would have had me directly heading for the rocks. The rocks were about 1ft below the surface.

From the DOP diagram for the day it can be seen that there are several points where positional accuracy is compromised. However - the time where the Lowrance and GPS tracks diverged was at 1425, and they converged again at 1448. The error is more likely multipath error than DOP.

GPS Errors, how to know if your data is OK.

Check your Altitude. !!

Lowrance SL2 files (saved as csv) and Garmin GPX files both save "Altitude" data in the datasets. When you are chugging around on a lake, mapping or fishing away peacefully, in theory your altitude should remain pretty constant.


GPS gives position data in 3 dimensions, and while positional innaccuracy is not obvious when just looking at latitude and longitude,  a wildly varying altitude is a very good indicator that something is wrong and that your GPS 2d position data is likely to contain errors.


Neither of the two datasets (Lowrance or Garmin) store DOP (dilution of precision) information, so it is almost impossible to analyse the data afterwards to find areas where the positional accuracy was out.


The graph above is a DOP graph for a 3 hour period in August when I was surveying a small lake. The blue line is from an excel chart of altitude vs time.


On the right the top table is a section of a gpx file during the period of good DOP.

The bottom table on the right shows the effect on altitude of a DOP of 3, as seen on the green part of the graph at the top of the page. It is the only indication in the data that something may be wrong.

I plotted this bad data, and had errors of up to 14m in varying directions during the period of high DOP. This gives a circle of diameter 28m within which the gps antenna could be anywhere. This is obviously not great for producing those perfect charts.

So if you are planning to get out on the water to spend a day mapping and produce that perfect chart - fire up Trimble Planning from Trimble Navigation - make sure it's loaded with the latest almanac, and print out a DOP chart. You'll see immediately when its probably going to be time to stop mapping and start fishing, or stop for a cup of tea.