Monday, 5 October 2015

Heading down the Pacific coast of Baja California? Create GPS-accurate electronic "charts" using Google Earth

On previous posts, we discussed various charts, official Mexico nautical charts, cruising guides, weather sources, and  communications. In the current post, we discuss using GPS-accurate electronic "charts" derived from Google-Earth images to supplement navigation resources when cruising the Pacific coast of Baja California.

March 2016: see also post concerning CM93 electronic charts.

THE PROBLEM: When we left Ensenada in November 2013 to sail down the Pacific coast of Baja California, we had our first encounters with navigating a coast where charts were often out-of-date, inaccurate, and lacking detail.

We had two guidebooks, both new (2013) editions (Charlie's Charts; and  Rain's Mexico Boating Guide). We also had paper charts in a Yachtsman's Chartbook, and CM93 electronic charts -- both of which we could see, before leaving, were lacking sufficient detail and accuracy. Importantly, in addition to these, we had an up-to-date C-MAP chart card for our Furuno chartplotter (also usable in our computer). In some places, the C-MAP card showed much better detail than the CM93 or Yachtsman's charts; in many other places the C-MAP chart were also lacking sufficient detail. In contrast with the excellent accuracy of the C-MAP charts in the USA and Canada, we had no idea as to the GPS accuracy of these charts in Baja California.

Two nights and two days after leaving Ensenada, wanting some sleep, we considered anchoring on the east side of Cedros Island at an anchorage called "La Palmita", indicated by both guide books. However, without any information on a chart or guidebook chartlet, zero detail on the C-MAP chart, and a depth sounding far off of what we were recording, we chickened out and continued on to Bahia Tortuga (Turtle Bay) -- losing about 1 hour of valuable daylight.

No detail and inaccurate chart for Isla Cedros' La Palmita anchorage: well within the "10 foot" depth contour, we still had >150' showing; it didn't help that the anchorage waypoint was showing on land. So we gave up and headed for Turtle Bay. (Interestingly, only a couple nautical miles south, the chart became very detailed (i.e., C-MAP included the new Mexican/SEMAR chart for Cedros town, produced in 2012.)

With daylight fading, we just made it through the "Canal de Dewey" (narrow channel between Punta Eugenia & Isla Natividad) before dark -- with so many lobster traps, so much kelp, and reefs, combined with currents, we would never do Dewey in the dark.

In contrast to most anchorages down Baja's Pacific coast, the C-MAP chart for Turtle Bay had quite a bit of detail. However, as we sailed towards Turtle Bay in the dark, we did not yet know whether or not there was a 1-2 nm offset (error) to the charts. Our radar proved to very useful that night. Nevertheless, it was not until the morning, when we were safely anchored and could clearly see all of Turtle Bay that we concluded the C-MAP Turtle Bay chart is indeed pretty accurate.  (Radar and charts, however, could not show the many lobster buoys at the entrance to Turtle Bay. We were lucky we made it through these in the dark!)

We did not have Navionics charts back in 2013. However, reviewing Navionics charts now (2015), we see similar accuracy between Navionics and C-Map charting for the Pacific coast of Baja. (In the Sea of Cortez, we have found that Navionics charts are not as up-to-date as C-Map; for example: Altata, Loreto, and, south of the Sea, San Blas.

The guidebooks provide many useful waypoints for anchorages. Although the location of these waypoints are indicated on the guides' chartlets, these chartlets are not usually very detailed (Charlie's Charts' chartlets appear to be handdrawn). So, one cannot be sure of the accuracy or location of these waypoints.

A PARTIAL SOLUTION: Reading other cruisers' blogs (see, for example: SV Totem; SV Ocelot; SV Valhalla; SV Soggy Paws), we came to a (partial) solution to the above chart inaccuracies and uncertainty about waypoints: creating GPS-accurate electronic "charts" from Google Earth (GE). Specifically, we used three excellent programs, all freely available: GE2KAP and Google Earth to create the GE-based electronic raster charts (each with .kap extensions), and the OpenCPN charting program to display the charts as well as the guides' waypoints. Although the GE-based raster "charts" do not provide depth sounding and other underwater data, one can clearly see land-based features (buildings, beaches, wharves, etc) and their relationship to the ocean, and usually, one can clearly see shallows, underwater spits, breaking waves, and often reefs. Moreover, entering the guides' waypoints allows one to see exactly where the waypoints are relative to the shore features.  Any offset in other electronic charts can also be determined. As with other electronic charts, one can use OpenCPN with a GPS to plot your vessel's position on the GE-based chart.

Unfortunately, we did not figure the above out until after we rounded Cabo San Lucas and reached the Sea of Cortez. Although we found the GE-based charts sometimes helpful in the Sea, they are not needed as much as there are excellent harbour/anchorage charts for the Sea of Cortez available from Shawn and Heather's Blue Latitude Press cruising guides, electronic waypoints, and electronic charts (also available for mainland Mexico, but not for the Pacific coast of Baja).

AN EXAMPLE: Abreojos, located between Turtle Bay and Bahia Santa Maria on Baja's Pacific coast, provides an excellent example of the utility of the GE-based "charts". The paper and electronic charts for Abreojos lack both detail and accuracy. The images below show what we had available as well as what we could have had if we had been using GE-based charts.

[After leaving Turtle Bay, we headed for Abreojos, with the intent of anchoring for a night. However, when as close as 10 nm away from Abreojos (we could see its main lighthouse), we found ourselves bashing into 10-20 kn SE winds. We figured these winds would blow directly into the exposed anchorage and, given the sketchy charts and numerous surrounding reefs, we changed course and headed directly to Bahia Santa Maria, another night and day away.]
The Yachtman's paper chart for Abreojos

Abreojos chartlet from Rain's "Mexico Boating Guide" Anchorage waypoints were given in the text.

Abreojos chartlet from Charlie's Charts: only the entry waypoint (for avoiding reefs) given.

CM93 electronic chart at maximum resolution. Waypoints from both guidebooks are shown, as well as the location of the lighthouse (from the Mexican "List of Lights")

The CM93 chart shows little detail and, assuming the waypoints are accurate, the electronic chart is clearly inaccurate (the main lighthouse is missing, the light on the hill is no longer in use, and the El Campo anchor waypoint is on land...).
C-MAP electronic chart showing Abreojos (at maximum resolution). Waypoints from both guidebooks are shown, as well as the location of the lighthouse (from the Mexican "List of Lights")

Navionics (2015): provides same (lack of) detail and inaccuracy as C-MAP (2015)

The C-MAP electronic chart provides a little more detail (although with very few depth soundings). Waypoints seem reasonably located. Again, the main lighthouse is missing or inaccurately located. (The lighthouse shown high up on the hill behind the town has long been out of use.)

The Navionics chart is much the same as the C-MAP.

Google-Earth "Charts":
Smaller-scale Google Earth "chart" (created using GE2KAP) overlaid on CM93 chart.

Google Earth "chart" (created using GE2KAP), zoomed in on the "Town" anchorage. Location of Rains' anchor waypoint now accurately shown relative to beach. Shallows and some reefs are also visible,

The town buildings and beach are clearly visible, as is the anchorage waypoint position relative to the beach, breakers, and the lighthouse.

Google Earth "chart" (created using GE2KAP), zoomed in on the "El Campo" anchorage (Rains). Location of anchor waypoint now accurately shown relative to beach. Shallows and some reefs are also visible.

As above, the Abreojos "El Campo en Medio" anchorage east of town is shown clearly on this zoomed-in larger-scale GE-based chart. Shallows (or reefs) are shown fairly close inshore of the anchorage.

We wished we had had these Google-earth based "charts" setup for our sail down the Pacific coast of the Baja peninsula. With the combination of the Rain's and Charlie's Charts guides, C-MAP charts, and the GE-based image charts, we would have felt far more confident navigating closer to shore. We highly recommend cruisers planning their trip down the Baja peninsula prepare GE-based image charts before heading south.

  • Before leaving Ensenada, try to obtain the new detailed large-scale Mexican charts from SEMAR (there is a chart sales office in Ensenada*). The reasonably up-to-date smaller-scale SEMAR charts -- all corrected to WGS84 datum -- would also be useful. (This may be easier said than done; we didn't learn about SEMAR until after leaving Ensenada. Ability to speak Spanish likely a huge help. Our attempt to obtain these electronic charts in La Paz was not successful, though we did find detailed paper charts.) Charts are the same price for paper or electronic (raster) formats. If you are bypassing Ensenada, you might consider a land trip from San Diego to Ensenada (bus travel is very easy). (But it would be a shame to miss Ensenada as it is a great cruising stop: easiest check-in to Mexico, excellent marinas, excellent provisioning (Soriana, Costco, etc), the best fish/shrimp tacos in Mexico, and a great bar (Hussong's).) 
  • If you are using C-MAP (or Navionics -- we've found they have similar issues; for example, see above, or this Latitude38 article on Navionics) chart cards, ensure they are up-to-date!
  • Obtain up-to-date cruising guides for Baja California. Enter their relevant waypoints into electronic form so you can transfer these to your chartplotter and your navigation computer/tablet charting program (easily done using GPSUtility) . (Also, you might be able to obtain trustworthy waypoints from other cruisers experienced with Baja.)
  • Using GE2KAP and Google Earth, create GE-image charts for each Pacific-coast Baja anchorage you expect to stop at (and consider alternate anchorages). In most cases, you will want a small-scale zoomed-out image chart as well as a large-scale, zoomed-in/detailed image chart. Load these into your navigation software, such as OpenCPN (these GE-image-charts will not likely be usable on your boat's dedicated chartplotter).
  • Use multiple sources of information while navigating down the Baja Peninsula; learn to use your radar.

*LOCATION OF CHART SALES IN ENSENADA: Estación de Investigación Oceanográfica de Ensenada, B.C.,  Boulevard Costero y Agustín Sangines (A & B), Ensenada, B.C. 22800 Tel. +52  (646) 172-5743   Email:

No comments:

Post a Comment