Author: jorgen

Serving your own PDOK maps

I am fascinated by navigation and maps. When I visit a new location one of the first things I do is getting a topographical map of the region. Sometimes I use historical maps, like the Werbata map (dating from 1911, see also Wikipedia) for my trips to Curacao. Since 2007 I log my activities (like running, hiking, cycling, swimming, surveying) using GPS (TomTom watch) and I like to plot these on a map online. Since surroundings change, I like not only to have the most current maps, but also historical maps. As a Dutchy a found the Kabouterbond maps worthwhile. Since 2012 the PDOK Basisregistratie Topografie (BRT) maps which can be downloaded for free in raster or vector format. PDOK is a cooperation between het Kadaster and the Dutch government.

So I end up with a collection of digital maps which I like to use with OziExplorer (for route planning e.g.) and online on my own websites. This post describes the way I proceeded to serve my maps for online usage.

PDOK based vector map from my server, described in this post

White balance underwater

A well known phenomenon to divers is the fact that water absorbs light, the red component in the light the most. This makes water appear blue or greenish. The deeper we get, the stronger the effect: it gets darker and it gets more blue. Diver eyes cope with it, cameras do not, as can be seen from the left picture. Therefore we need correcting the whitebalance, so we end up with something presentable.

OziExplorer maps from GeoTIFF


I still find Oziexplorer an excellent and versatile program for navigation, despite of the many, many apps relying on on-line maps.

This page describes how to create Oziexplorer maps from GeoTIFF, including map merge. As an example we take the Basis Registratie Topografie (BRT) Top25raster 1:25.000 maps of the Netherlands that can be downloaded from PDOK and we will create an ozfx4 map covering the entire Netherlands.

The GeoTIFF format is an image tif format that contains geo information.

Window of opportunity

When taking wide angle photos underwater we observe a remarkable effect: if we look up in quiet water, it appears that we can look at the world above water through a circular window.

Circular window

The reason is found in Snell’s law, which states that light is refracted when going from one medium to another medium with different ‘refraction index’. In our case light goes from air to water. Light propagates not in a straigt line, but is refracted.

Natural light photography underwater

Flash, Forrest, flash!

Many starting photographers conclude there is not much light underwater and start flashing. Many using one flash, more luxury divers use two flashes to prevent shadows. Lazy photographers close the diaphragm and blast out a hell of flash light, having always sharp photos due to the large depth of field and not having to worry about white balance. This however, has some drawbacks:

  • Flashing casts shadows
  • Flashing illuminates dust between camera and object, if flashes direction is not set properly
  • Many fish flash back by reflecting your flash
  • Backgrounds become dark, making the image flat



Flashing casts shadows



Backgrounds appear dark


Light comes natural

Photographing using natural light has advantages, when done properly:

  • Light underwater is diffused by the water, even in sunny conditions giving nice and smooth illuminated objects
  • You can use the color of the water to get the real underwater experience
  • Dust becomes less annoying
  • You capture much more of the environment, since everything is illuminated, not just your subject


Naturally illuminated pike

Open Sesamy: Port-knocking using just one port

Port knocking is a mechanism to provide additional security to firewalling. Port knocking is normally used in firewalls. A target port needed for a particular service (e.g. SecureShell, SSH) is normally closed and the service is unreachable. Only by ‘knocking’ on a number of ports in a particular order opens the target port. Knocking means: sending a TCP or UDP packet to the port. Though the knock ports are closed and won’t respond, the firewall notices the knocks. To the outside world the firewall appears totally closed. Portscans fail. Only when the proper sequence of ports is knocked the target port opens and the service is reachable for a few seconds. During this time-span the service can be reached.

There are a large number of port knock implementation. See for an overview: portknocking.orgOpen Sesamy differs from other mechanism because only one port for knocking is needed. It may even be the target port. It is implemented using Linux iptables. This is another difference: most port knocking algorithms are implemented as scripts scrutenising the log files. Using iptables the packet filtering is exploited as present in the Linux kernel or modules.

Photographing the Milky Way


At some dark parts on this globe we have a beautiful view on the milky way. It shows as a lighted band on the nightly sky. It is possible to capture it on camera. The fun part is: The camera can see more than the naked eye.

It requires a SLR on tripod and a bit of technical knowledge. This shows how easy it is. Hardest part is probably to find dark night sky, real dark night sky.


More info on

Solar Power

The gauges present the actual production at my home: left the Photo-Voltaic (PV) System, right the MicroCHP production. More information on the project site.