dataglobalnavigationwaterways
I'm looking for a free high resolution map that clearly indicates the land and sea in the world. I need it to calculate distances over sea between ports.
Additionally it should also include inland waterways and canals.
Does anybody know whether this data is freely available?
I'd suggest to have a look at TomTom offer. I've used their MultiNet product. This dataset is mainly developed with navigation in mind, but some of the layers provided cover administrative division and zip code areas.
At least 10 levels of Administrative Orders are defined for each country. The highest level, Order 0, is always the Country, which is made up of all lower-level units. Order 8 is also always present. The name of this level and the extension of its units are country dependent. Order 8 is the lowest level that covers the whole territory of the country (source).
I only used data for Switzerland but they seem to maintain global coverage:
I cannot comment on pricing and licensing of the product, but perhaps you could get a deal of getting access to admin boundaries only (which, if possible, could hopefully reduce the price).
I've found two sources that appear to provide easy to read explanation between Radiance vs. Irradiance and remote sensing reflectance and water leaving radiance.
Starting with Radiance vs. Irradiance:
Irradiance is simple: exchange of energy (in the form of photons) across a given area of flat surface per time. Radiance is more complicated: exchange of energy (in the form of photons) across a given area of flat surface per time and then divided by the amount of steradians from which the "given area" is collecting light.
The author uses two figures to discuss Radiance and Irradiance with respect to Illuminance.
Also, a good reference may be The Light Handbook provides additional information.
In terms of remote sensing reflectance and water leaving radiance
This comment here
Remote sensing reflectance (Rrs) contains the spectral colour information of the water body (below the sea surface). Rrs is the ratio between water-leaving radiance (Lw, above the sea surface) and downwelling irradiance (Ed, above the sea surface). Lw can be estimated from above-water radiometric measurements, in this case reflected skylight must be removed using a "surface reflectance factor" (rho).
Further comment: In ocean colour, often the term "water-leaving reflectance" (rho_w) is used instead of Rrs. The difference is: rho_w = pi*Rrs (with related change in units)
Illustration of light rays contributing to the irradiance reflectance
Illustration of light rays contributing to the remote-sensing reflectance
Best Answer
One of the best sources for open-source vector and raster map data is Natural Earth. There is a variety of data available at large, medium and small scale. You would likely need the Ports dataset available from 1:10m Cultural Vectors and the Rivers, Lake Centerlines available from 1:50m Physical Vectors.