Author: jorgen

KeePass, Keeweb with OneDrive and Dropbox

by jorgen on 2023-01-11 in Tech&Science

Introduction

This blog describes some hints to get a self hosted Keeweb working with a KeePass database files stored on Microsoft OneDrive or Dropbox.

KeePass is an open source password manager. It is a PC application storing your credentials and secrets in an encrypted .kdbx database file. I use it for some time now and I am happy with it. I started to use it with my database on a memory stick, to have it anywhere I go. This, however, is not a fine way of working, because I tend to forget my memory stick. So I wondered if I could use my KeePass database in a self hosted web based application. I did some searching around and found Keeweb to be most applicable.

Keeweb comes in a desktop application as well as a web application. The latter seems to be about what I want.

However, I like to be in control and host the application myself. I finally succeeded to get it working with my KeePass database on Microsoft OneDrive and and on Dropbox. There are two sites I would like to mention that provide a howto.

Host Your Own Multi-User Password Manager with KeeWeb and WebDAV
Dropbox and GDrive, which also explains Microsoft OneDrive

Why then, this blog as third howto? Well, following both links still gave me some head-aches. Therefore I report my findings

Mercator projections, a comparison

by jorgen on 2022-11-03 in Navigation

Introduction

While writing the previous post on Rijksdriehoeksmeting and OziExplorer, we came upon two map projections: Oblique Stereographic and Transverse Mercator. In this post we will have a look at the Mercator and Transverse Mercator projections and try to compare. Both projections are quite frequently used. So let’s have a look at them.

Mercator projections are cylindrical projections. The earth, represented by a spheroid or ellipsoid, is projected on a cylindrical surface. In contrast to a sphere, a cylinder can be flattened out as a 2D surface, the map.

The page is supported by a Java code project MapDatumConvert on Github in which I have programmed the projections and Map Datum conversions between Rijksdriehoeksmeting and WGS84.

OziExplorer and Rijksdriehoeksmeting

by jorgen on 2022-09-26 in Navigation

Introduction

OziExplorer is my favorite tool for working with topographical maps. Even in this era of mobile apps with all kind of online maps, I am a bit old fashioned by having my own collection of maps and OziExplorer is a great way to use them.

I am a Dutchy and here in the Netherlands the ‘authorities’ that are responsible for Dutch topographic maps use the Rijksdriehoeksmeting (RD) Map Datum.

*Primary reference points of the Rijksdriehoeksmeting Stelsel*

In this blog we look at the Dutch maps and describe how to get them properly working in OziExplorer.

Serving your own PDOK maps

by jorgen on 2021-05-14 in Navigation, Uncategorized

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

by jorgen on 2021-04-16 in Uncategorized

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

by jorgen on 2021-04-09 in Navigation

Introduction

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

by jorgen on 2017-07-14 in Photography, Scuba diving

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

by jorgen on 2016-06-29 in Scuba diving

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

by jorgen on 2014-05-02 in Tech&Science

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.org. Open 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.