AVANTE Hard Aground!
It was mentioned when we were rounding the headlands off Punta Mita that the navigation charts for Mexican waters leave a lot to be desired. Both The Captain and The First Mate have decided they are mostly useful as a general guideline, but they are definitely not to be relied upon for precise navigation. Many of the charts are based on underwater surveys that were done by a US Navy ship well over 100 years ago!
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Look at the dates on this “current” chart we purchased for our sailing trip to Mexico. 1873 to 1901? The First Mate thinks this chart belongs in a museum rather than in every day use on an ocean-going boat!
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Granted that, unless subject to seismic activity, not much changes underwater in 100 years, there remains a lot of data (like rocks that have been found or could be found with modern equipment) that ought to grace charts used by mariners today. Modern mariners would greatly appreciate such information, but that apparently does not concern the Mexican government.
An interesting phenomenon occurs when one needs to find one of the new marinas that have been built in the last 100 years. Many have been placed in dredged-out estuaries or even dug out of the land. These marinas naturally do not show up on our 100-year old charts, and no attempt has been made yet to locate them for the searching mariner. We have been in two already which are not on our charts: Mazatlan and Paradise Village Marina. To find these marinas, the official Mexican charts are useless. We have to rely on our guidebooks and the GPS waypoints they list for the marinas.
A further interesting discrepancy occurs between our charts and our actual location determined by GPS coordinates. Our actual GPS position can be displayed on the Mexican charts on our computer and on our chartplotter, but there is a big difference between where we actually are and where the chart displays show that we are. At the end of each day as we head shoreward to anchor, we often end up “hard aground” on the chart displays. Night after night, Avante shows herself to be an agile and adventurous amphibian, for the displays clearly show that she has crossed the beach and anchored well inland. The First Mate feels that Avante is turning into something like Noah’s Ark. Which mountain are we going to alight on tonight?
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The chart shown here is what shows up on our computer. The 3 shades of Blue are water. Gold is land. The Red line is Avante’s actual GPS track. The Yellow dot near the town of “Ipala” is Avante anchored well up in the surrounding hills. Look at the light blue area. The circles are rocks through which we supposedly motored.
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The reason for this goes back to the inaccurate Mexican charts. When anchored in Punta Mita the charts are about 1 1/2 miles off from our actual GPS location. The Captain decides one evening that he is going to tackle this discrepancy and try to adjust our chartplotter to compensate for this error. Old charts are usually pretty correct on latitude. It was the longitude that they had trouble with until they had better chronometers. It takes some figuring and finagling of buttons and controls, but he finally gets our chartplotter to compensate for the inaccurate longitude. The adjusted chart now shows us anchored close to where we actually are. This is a relief! The next morning we take off. Things look good until we get about 20 miles down the road and find that the actual location of nearby land is not where it shows on The Captain’s meticulously adjusted chartplotter. We conclude that the problem is that the charts are blissfully and inconsistently inaccurate! They don’t just need a longitude correction. The only thing reliably consistent about these Mexican charts is their variable inconsistency. To deal with this, The Captain would have to reconfigure the data every time we changed our location, and that is just too time-consuming and frustrating. We are just going to have to put up with this misguided visual image and do our own mental math to navigate Avante.
The First Mate must give the Mexican government some credit. In a few cases, where navigation need would appear to be most urgent, updates and revisions to the charts have been made. For instance, the navigation chart for the main harbor in Puerto Vallarta has been updated. Cruise ships use this harbor so the Mexican charts had better be accurate. We motored in there for fuel prior to heading south to Manzanillo. The harbor is tight once one is beyond the entrance, and with their updated chart, we had no problem. Our chartplotters correctly displayed where we were in the harbor. Accurate chart position we may have had, but we still carefully monitored our depth meter and hoped that the channels had been kept dredged.
For the last three years, we have been sailing with US and Canadian charts in some very remote areas with tricky navigation issues. US charts are very good, and Canadian charts are superlative. All have been updated using modern tools like GPS and satellite photos to accurately determine the position of things. If a series of rocks and shoals was ahead of us, their location was right on, and we could confidently steer through and around them. If a depth was listed, we knew we could count on it to be accurate. Not so here in Mexico. How we miss those so very accurate charts!
Fortunately, cruising Mexico does not hold the navigation challenges that one encounters in the Pacific Northwest. The Captain jokes that most of the time here in Mexico you only need to stay off the beach. He says that you can sail well off the beach for hours without having to worry about anything. Every 50nm or so along the coast, there is an offshore rock that you need to avoid. Navigation is not very hard here, and consequently, the inaccurate charts are not much of a problem on the west coast of Mexico.
Avante may show up “hard aground” on our chartplotter every night when we anchor. We can live with that. To be on the safe side and avoid really grounding, The Captain normally keeps Avante at least 2 miles off shore. When our books talk about a rock 1 1/2 mile off shore or they say to give a point a wide clearance, we on Avante double the distance to steer clear. The only place that we want Avante to be “hard aground” is on the chartplotter display.