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Author
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Topic: Importance of chart datum
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David Burch
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posted May 19, 2006 09:20 PM
On our recent training trip to Alaska we ran across a good example of why it is important to match the chart dautm of the GPS with that of the chart you are using. In this case, we were using echarts, and they were rather old.
The echarts were of a vintage made on datum NAD-27, whereas we routinely have the GPS set to WGS-84, the most recent datum and most useful in general.
When the datums do not match, however, you can end up with your boat plotted on the wrong place on the chart. We have an example below.
We were preparing to anchor in this cove and noted that the echart location of the boat did not agree with the radar. This chart showed us way over on one side, which would infact uncover at low tide (see next picture), whereas the radar showed that we were actually in the middle of the cove.
In the picture above, the GPS was set to WGS-84 but the chart was NAD-27.
The picture below is from another computer and GPS system that had GPS and chart datums both set to WGS-84. The mark shows where the anchor was dropped.
The radar image shows us truely in the center of the bay.
The pictures below compares what we saw after the GPS on that computer was set to NAD-27 to match the chart. In both pictures the echart is one made to NAD-27. The left has GPS set to NAD-27 and the right has the GPS set (incorrectly) to WSG-84.
Below we use the boat to cursor range display to see that these two positions differ by 0.063 nmi which is 126 yards. Not much in many cases, but an amount that could be crucial in close quarters. Needless to say, the extreme value of radar in spotting this problem is obvious -- though in this case it was light out when we anchored so we could see the problem as well. At night or in the fog would be another issue.
Always keep in mind to check that the datum of the GPS matches the charts in use, especially when the charts are old.
From: Starpath, Seattle, WA
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