Monday, January 20, 2014

ECS Without GPS

We might assume an electronic charting system (ECS) is only useful if we have a GPS connected to it so we can track our boat across the chart. But the main component of the system is actually the echart navigation software and a set of echarts.  The GPS is just a luxury. We can do a tremendous amount of sophisticated navigation, both easily and accurately, without the GPS.

In fact, the crucial part of a well-navigated voyage is done before the boat pulls away from the dock. We can do a rough layout of the route for quick time and distance estimates, and then zoom in along the route and fine tune each of the waypoints to optimize their use underway.  This usually calls for choosing them such that they correlate with good visual or radar targets along the route. Then give each a name, not just a number.

These programs also have built-in tide and current predictions, so we can estimate SMG (speed made good) along the route, and then print out a route plan with individual route-leg courses and distances and ETAs all tabulated. This is prudent preparation, with or without GPS to track us across the chart.
All US echarts are free, so now there is no reason not to have all possible charts of interest at hand for any route. (I might add that the full-chart pdf versions new this year are quite remarkable. They can be viewed in a tablet or computer without any special ECS software. Check out for a portal to the echart options.)

In this note we look at a couple ways you can navigate with echarts without relying on GPS–or put another way, if you happen to lose your GPS, don’t shut off the ECS. It remains your most convenient and accurate means of chart navigation.

Consider the standard challenge of getting from A to C, without hitting B, an unmarked underwater hazard, in the dark, in strong current. Consider a hypothetical example, where we must enter a channel about 3 miles to the west at night, when we have read in the Local Notice to Mariners that the buoy “FR” marking the reef is off station and not showing its light at the moment. (Quotes indicate what is actually painted on the buoy.)

We look at a couple ways to monitor progress (without GPS) to be sure we do not get set down upon the reef. Tidal current predictions are for 1.5 kts setting toward 135 T at the time along this route. We can learn the currents directly from the ECS program underway, without wireless connection.
These are belt and suspender conditions. We want as many safety checks as possible, and we want to monitor at least two of them at all times. We start from a known position having just passed a small islet, now our job is to get over to the channel along some efficient but still safe route.  We can power at 6 kts in these conditions, but we know we will be set by the current and need to correct for this. To track due west, we will need to point into the current. 

A quick approximation of the correction is 10º, as noted in the sidebar 3 (at the end), but with an ECS at hand, we can very quickly be much more specific as shown in sidebar 1. With this method we confirm that the correction is 10º and also learn that our SMG as we proceed will be about 4.9 kts (Figure 1.) 

Figure 1. An ECS solution for course to steer to make good a desired direction.
So as our knotmeter reads 6 kts and our heading is 280 T, we can assume we are making good 270 T at a speed of 4.9 kts. This helps us keep track of our position by dead reckoning as we proceed, but we need ways to check this.

 1. Current Corrections from  ECS.

Finding the course to steer (C) to make good a desired direction (CMG) in the presence of current (Set, Drift) is a vector problem that is easily solved with an ECS display. We do it by making a route that has 3 legs, zoomed in or out on the chart to create a scale of 1 nmi = 1 kt.

Plot the route (see Figure 1).

Leg 1.  Draw a leg (A to B) in the direction to be made good (CMG) with a length longer than the knotmeter speed (S) you can make in present conditions.

Leg 2. Draw a leg (B to C) into the direction the current comes from (Set + 180) with a length equal to the Drift.

Leg 3. Draw a leg (C to D) that takes you back to the line A-B with a length equal to S. This is done in two steps, first back to the line, then slide up and down the line until the C-D length is exactly equal to S.

Read the results.
(1) Reverse the leg C-D (often just right click, and choose Reverse) so you can read the course to steer that will make good A to B.

(2) Measure the length of D to B to find the SMG you will make when correcting for the current. This can be done with a Range and Bearing tool, or just move A to overlap D and read the new leg length.
Example (Figure 1).

We want to make 270 T, the current is 1.5 kts in direction 135 T. We find the C we need is 280 T, and the resulting SMG will be 4.9.  Our quick estimate was right on in this case, but ± 5º or so might be more typical.

Next we can in seconds measure from the echart an accurate danger bearing to the QG (Quick flashing Green) light that will keep us off the buoy (“FR”) that should be marking the reef but is not in this hypothetical example. We see that this bearing should not ever be larger than 254 M (we are assuming a 20º E variation).  We watch it with a bearing compass as we proceed, after first checking that it is now due west (250 M) as it should be. If the bearing gets smaller we are slipping north of our track; if it gets larger we are getting pushed south. If it gets larger than 254 M, we are headed for the reef, as shown in Figure 2.

Figure 2. Selecting a danger bearing to the QG light dead ahead at the start. The 0.5nmi range rings mark distance from the starting point, which helps for quick chart plotting from log readings.

In this hypothetical situation, the only thing we have left to use is our sounder. This is not an ideal route for depth sounding navigation, but even in this case there are still a few guidelines that can assist our navigation. The main point for now is to show how powerful the ECS is for setting up  sounder navigation. The details are in sidebar 2 and Figure 3. To do this accurately we need tide height, which we also get from the ECS program with the click of a button.

Figure 3. A line of soundings made from an ECS program. We added the plot at the top to illustrate the pattern. It was made by copying the route plan and pasting into a spreadsheet and plotting. The range rings at 0.5 nmi set on the starting point helps monitor the correlation between depth and DR position. Though not the best bottom for sounding navigation, we still have a few things to note. First the depth should drop off rapidly, then carry on slowly deepening. If it should go up at about 0.5 - 0.7 nmi off (marked red in the plot), we know we are getting pushed south. Then if we are right on track, we should see the 19-fathom bump at about 1.7 nmi off (marked green in the plot).
    This is just a sample of the information we might obtain by this type of analysis, which is very quickly implemented with an ECS program.  We used Coastal Explorer from Rose Point, which is well suited to chart navigation with ECS, as are other leading programs as well.n

 2. Line of Soundings from ECS

Whenever the water is not too deep (i.e. our sounder works) and not too flat, we can usually gain much nav info from a line of soundings. Here is a fast and accurate way to set this up using ECS.

(1) Set up the route line for the leg to be navigated.

(2) Then zoom in and follow it along, inserting a waypoint at each depth contour or notable sounding.  Name the waypoint with the depth.

(3) Then use your ECS function that itemizes the route. It has various names: Route Details, Plan Book, Voyage Plan, etc. It is a list of waypoints, showing name, location, and the range and bearing between each one, and ETA assuming some SMG. That list shows exactly what depth you should read after traveling the indicated distance. An example is shown in Figure 3.

(4) Then as you proceed, you can record the depth at various log readings, and compare what you see with what you expected. In many cases you can find an approximate position from this comparison. Tide height and sounder draft must be accounted for. With ECS you can set range rings on your departure point to show where you might be as the log reading increases.

With paper charts you can draw a line on transparent paper in the right orientation, and mark off the measured depths using the same miles scale as the chart.  Then slide the transparency around till you match your measurements.

These are just a couple chart navigation exercises that are nicely solved by ECS without GPS. There are many. Besides the range and bearing tool for standard plotting, a versatile option that is often overlooked is the ability to set multiple range rings on any mark or waypoint. This has numerous applications, we have shown only one.

The navigation procedures described here are adapted from our new book Inland and Coastal Navigation, 2nd Edition.

3. Approximate Current Corrections

With the bow pointed toward your destination, define the relative current direction as one of the following.
Dead ahead  (or dead astern): 
Set angle = 0º

On the bow (045R or 315R):
Set angle = (Current/Speed ) x 40º

On the beam (090R or 270R):
Set angle = (Current/Speed ) x 60º

On the quarter (135R or 225R):
Set angle = (Current/Speed ) x 40º

Then assume the correction to make equals the set if none were made. 

That is, if you figure you would get set by 15º, then point 15º into the current to track along your desired course. There are several approximations involved here, but we rarely know the details of the current precise enough to justify more analysis.



Unknown said...

The post says: "Tidal current predictions are for 1.5 kts setting toward 135 T at the time along this route." However, it looks like the set of the current is 315 T in Figure 1. Is 135 a transposition typo for 315?

Unknown said...

Never mind, I see that 315 T and 135 T are actually reciprocals.