Originally Posted by jpemb7
Just be aware that some older scanned charts are not as accurate as current
U.S charts. I'm sure it has been said before but when you are looking at that little green boat on your laptop
you can be lulled into thinking it represents the real world. Some digital charts are very, very accurate. Others are not.
It is important not to be complacent on this one.
Yes and No - In order to find out the actual date of the data contained on the chart you need to be able to read the Chart Title area which is available only on Raster e-charts. Raster e-charts are actual scanned images
of the real paper chart (actually the acetate, if available). This includes all the borders, notes, legends and chart title. Vector charts do not include this data on the actual displayed chart.
- - In the title block will be the dates of the surveys and soundings. Which might surprise you. For instance, the 21005 chart of Mexico (Baja California) was last surveyed in 1890 and partially updated in 1961. That is the "newest" chart. All the chips, CD's etc. are using this chart regardless of the date on the CD/chip/etc.
- - Within the USA territorial waters the NOAA charts are updated for major commercial
harbors whenever a significant change occurs due to channel movement or relocation of buoys. The vast majority of other areas are (as stated in the NOAA disclaimer you must acknowledge) an average of 40 years or more old. Topographical and soundings data in non-commercial harbor areas is grossly out of date.
- - When it comes to the chart datum, that can vary from WGS-84 (most common GPS datum) to any of the other 134 different datums in use in the world. The USA alone has 24 different datums in use for charts. Average difference between datums inside the USA can be up to 1.5 nm. Somebody has to pay the salary of the cartographer to revise a chart to the standard WGS-84 and for non-commercial harbor areas this is not considered a necessary expense by the government
- - Purely by chance some charts will be "dead-on" and others will be off either by inaccurate survey
or the use of another datum. One advantage of the SoftChart series of e-charts is that they can be easily manually "re-registered" if necessary if you know how to do it. BSB's cannot. Vectors, no way.
-- So the last sentence by jpemb7 is critically important - do not be complacent. Any chart is merely a "guide" to an area, it is up to you to use your eyes, ears, other aids such as radar
to determine where and what is really out there. More than a few times my "little boat" on the screen
is merrily sailing through a mountain while in reality I am in the middle of a harbor. This is known as "offset" which was adjustable in the old Loran
systems. In GPS systems this is only available by matching datums between the chart and the GPS or re-registering the chart.
- - Google Earth screen
captures and be converted to graphics and by various "MapCal" type programs can be converted into reasonably accurate harbour charts but unfortunately do not have soundings or some sub-surface obstacles that are drawn on actual nautical charts.