Reprojection of Raster Charts

March 17, 2011

Most raster charts for marine use are created in a convenient and universally accepted Mercator projection. Mercator projection allows charts to be “quilted” (or connected together on a world-wide view). However, some raster charts still use other projections for reasons of convenience, tradition, data availability etc.
These charts can’t be directly “quilted” with the rest of Mercator-based “world”.

In US, NOAA uses non-Mercator, primarily Polyconic projection raster charts, for coverage of Great Lakes area. Polyconic was a predominant projection for US charts from mid-1800s to early 20th century.

To fit non-Mercator chart into Mercator world, it needs to be re-projected, that is – chart image needs to be transformed to match Mercator projection requirements.

While differences may not be as clearly visible on a large scale (small area) chart, small scale charts are very telling. Here is an example of small scale chart (NOAA 14500, Great Lakes overview) in original Polyconic and reprojected Mercator.
14500 Polyconic

14500 Mercator
Note how parallels and meridians are not perpendicular on the Polyconic projection chart above and intersect under 90 degree angle on the Mercator chart below. Mercator projection is conformal, i.e. – able to preserve angles and courses, including those between latitude and longitude lines.

Chart reprojection is available in current release of PolarView NS.

Navigation / Instrument Acronyms

February 22, 2011

Since users ask about these from time to time, here is the list of acronyms used to denote certain navigation related data items in PolarCOM instruments and elsewhere:

POS – position, that was easy
SOG – speed over ground. Speed of a vessel relative to stationary land.
COG – course over ground. Course of a vessel relative to stationary land.
VTW – speed (velocity) through water. Speed of a vessel relative to water.
HDG – heading. Compass direction into which the bow of a vessel is pointing.
DBK – depth below keel. Water depth measured from bottom of keel
DBS – depth below surface. Water depth measured from surface of water or vessel waterline.
DBT – depth below transducer. Depth as returned by transducer, unadjusted.
AWA – apparent wind angle. Angle of apparent wind as seen from a moving vessel, relative to vessel bow-stern axis.
TWA – true wind angle. Angle of true wind relative to vessel bow-stern axis.
TWD – true wind direction. Compass direction from which the true wind is coming.

VMG – speed (velocity) made good. Speed with which a vessel is moving towards its destination, as measured on a line between current vessel position and destination.
XTE – cross track error. In general, shortest distance from current vessel position to the nearby route leg.
BTN – bearing to next. Compass bearing to the next waypoint enroute.
DTN – distance to next. Distance to the next waypoint enroute.
ETEN – estimated time enroute to next.
ETAN – estimated time of arrival to next.
DTD – distance to destination. Distance to the final point of multi-leg route. Measured along the route.
ETED – estimated time enroute to destination.
ETAD- estimated time of arrival at destination.

ROT – rate of turn.
RDA – rudder angle
SDA – set and drift angle. Used in combined set & drift instrument.
SDD – set and drift direction. Used in combined set & drift instrument.

Any of the direction acronyms above may have a suffix of ‘M’ to denote that they are in degrees magnetic. Without this suffix, assume degrees true. I.e. HDGM vs. HDG.

Happy New Year – 2011!

December 31, 2010

Dear Polar Navy users, friends and chance visitors to this blog!

I’d like to wish you a happy, peaceful and successful new year! Let your routes be free of trouble, your arrivals timely and let wind be always in your sails wherever you may go!

Downloading NOAA Charts from PolarView

November 26, 2010

NOAA is getting with the times and now you can find out about NOAA chart updates on Twitter here:

PolarView users can download and update NOAA charts directly from the program. Just follow these step by step instructions:

1. Open “Download” dialog by selecting “File-> Chart Download” menu or using keyboard shortcut Ctrl + ‘D’ (Windows) or Cmd + ‘D’ (Mac).

2. Press “Select” button to select area on screen.

3. Click on the first point on a chart. A yellow selector box should appear. Move the mouse to resize selector. Click on a chart again to stop resizing selector box and finalize selection.

4. In “Download” dialog click “Find Charts”. A new window should open, as shown in the image below, presenting you with a choice of available ENC charts for selected area.

Chart in the list will have different color depending on current status, also shown in status column. Charts that are up to date will be shown in blue. Charts that are out of date and have newer version available, will be shown in red. Charts that have not been previously downloaded will be shown in black.

5. Select charts you would like to download or update. To select a block of charts, click on the first entry, scroll and Shift-click on the last chart entry. To select multiple individual charts use Ctrl + click (Windows) or Cmd + click (Mac).

To update existing charts, sort by “Status” column by clicking on column heading. Once sorted by status, you can select only those charts that are out of date or not installed.

6. Press “Download” button to download and install selected charts.

That’s all there is to it 🙂

Chart Update Window

New chart info display (under construction)

November 13, 2010

It’s been quiet on this blog for a while, but not for lack of development. I am working on a number of new features for the next release (probably early next year, though specific timeframe may change).

One issue that many users commented on, and which was neglected until now is chart information display. While S57 chart info displayed by PolarView was comprehensive, it was not particularly easy to read.

This is finally changing now. A new chart info display is under development. It will combine most important data in a single screen, while omitting more technical items (such as what scale to show a given object at). An “old style” comprehensive list will continue to be available, selected as a configuration option.

Here is a screen capture of what a new chart information dialog may look like.

New vector chart info

I found that with this display I am constantly surprised by how much information is in S57 vector charts, much of it unnoticed previously.

That said, this is work in progress and I am interested in your feedback. What do you like about it? What would you change or improve? What would be good extensions to this display to consider in the future?

Fun with ENC charts

August 29, 2010

Looks like NOAA may be using automated conversion software to create ENC charts from satellite images. It works great most of the time, but sometimes things don’t go as planned. In this chart (US5NY43) Mid Hudson Suspension Bridge seems to be charted based on a shade of the bridge rather than bridge itself, moving it somewhat North. Stated accuracy of 1:40000 scale chart is about 140′ so this one is probably just about within the limit.

Good navigators always remember that chart is not a substitute for a proper lookout.

P.S. A bit more on this – the following is my guess, correct me if I am wrong.

So, NOAA may be trying to do the right thing. Note that the satellite image is taken from position slightly due North of the bridge, rather than vertically. From that point of view bridge as tall as this one (134 ft) would be visually shifted South relative to anything at “sea”/water level. On the other hand, if the sun was at high noon and directly overhead, shade from the bridge would be precisely under it at exactly the sea level, and would provide a perfect outline of the bridge on a chart. I think this method works “most of the time”, in particular for images taken during summer months in Southerly latitudes.

Here we have a relatively high Northern latitude, and photo likely not taken at noon, so the sun is low in the southern sky. Because of that, the shade is located North of the bridge. The “real” bridge position should probably be charted in the middle between the shade and the bridge on the satellite photo.

Quick Video Demo

June 2, 2010

Below are a few videos demonstrating some of the basic functionality of PolarView. I did not spend much time on it so quality is not great. I still need to figure out how to annotate mouse clicks and keystrokes.

Some of the features in the video are not yet available in the current release – but will be in the upcoming version.

Demonstrating how to lay a route:

Miscellaneous options including:
– Setting and moving a waypoint
– Changing palette colors
– Showing tide information for a given tide station
– Searching chart list for a specific chart

Using a GRIB viewer:

A better demonstration of a GRIB viewer (using an older version of PolarView), made by a user. Thank you!