ArcHydro : 2- Preparing a corrected DTM for hydrology – Part 1

A  Digital Terrain Model is a representation of the elevations of a territory. Each cell (pixel) of this DTM contains a height value. According to the generation means used for the surface and the allocated size ofr the cells, the height assigned to each one is more or less close to the exact reality.

If you want to use the DTM for a 3D view of the territory (ith ArcScene, for example), you can use it as it is and without any special precaution. By cons, if you want to model the water flow on the surface of this territory, the first thing to do, and the most important is to correct and adapt it to this objective.

What you will be doing during this step will define the quality of the results obtained regarding the watersheds and the different hydrological calculations available. Continue reading “ArcHydro : 2- Preparing a corrected DTM for hydrology – Part 1”

Hydrology with a GIS, for the Dummies (that we are): calculation of the flow (4)

In the last three articles, we have discussed an example of application for each of the main methods for calculating the flow of a watershed. Although they are quite self-explanatory, we must go through a little theory to complete the subject.
First of all, let’s see what happens with the most widespread method, mainly used for all ArcGis hydrological calculations, the D8 method. Continue reading “Hydrology with a GIS, for the Dummies (that we are): calculation of the flow (4)”

Hydrology with a GIS, for the Dummies (that we are): calculation of the flow (3)

In the two previous articles ( Hydrology with a GIS, for Dummies (that we are): calculation of the flow (1)   and   Hydrology with a GIS, for the nulls (that we are): calculation of the flow (2 ) we have approached three methods to calculate the flow of flow: two one-dimensional methods (D8 and Rho8) and a two-dimensional method (MFD).

Now we will discuss a variation of the most widespread method (D8) which we addressed first. Continue reading “Hydrology with a GIS, for the Dummies (that we are): calculation of the flow (3)”

Hydrology with a GIS, for the dummies (that we are): calculation of the flow (2)

This article is the continuation of the previous article:  Hydrology with a GIS, for dummies (that we are): calculation of the flow (1)

Now, we will discuss the result of the application of another flow calculation method.

Second method: Rho8 Continue reading “Hydrology with a GIS, for the dummies (that we are): calculation of the flow (2)”

Bivariate display with ArcGis

It is not easy to visualize a bivariate map with ArcGis , i.e a map where the entities are categorized according to two variables located in two independent layers.

The colour assigned to each entity responds to a colour scale along two axes, X and Y:

Therefore each layer must be classified according to four classes. The entities which belong to the first class of each of the layers share the colour located at the bottom and left, those belonging to the strongest class of each layer will have the colour located at the top and on the right. Continue reading “Bivariate display with ArcGis”

How to integrate free marine and terrestrial weather forecasts into QGis

In the previous article we discussed How to integrate free marine and terrestrial weather forecasts into ArcGis.
In this article we will discuss how to integrate weather forecasts into a QGis project . We will use marine forecast data for 8 days. These data is provided by the companies   NASCA   and   ACTIMAR , via the products  enav-forecast . Besides the software that employs and shares these forecasts for consultation and analysis ( enav-viewer ,   enav-forecast ,   enav-forecast for iPhone ,   enav -Pilot ), these companies also provides you with access to data via a WFS connection.

What data is available? Continue reading “How to integrate free marine and terrestrial weather forecasts into QGis”

Hydrology with a GIS, for dummies (who we are): calculating the runoff (1)

A quote to start this article:
”   In the same way that a text editor’s manual does not teach you how to write a short story or a CAD tutorial does not tell you how to calculate the size of a beam for a building, this guide does not teach you anything about spatial analysis. Instead, it will show you how to use the QGIS Processing Environment which is a powerful tool for performing spatial analysis. It is up to you learning the concepts that will be essential to understanding this type of analysis. Without them, you will not be able to use the environment and its algorithms even though you may be tempted to do so.   »( QGis documentation )

In this series of articles, we will not try to teach you hydrology. We will just open a few doors, usually closed. If this does not teach you hydrology, at least we hope to sow the doubt about what you are doing, doubt that will lead you to learn (if you wish to do so). Continue reading “Hydrology with a GIS, for dummies (who we are): calculating the runoff (1)”

Replicating a postgresql database in a windows workstation

We will configure a logic   replication of a database table of our linux server in a postgresql installation in a windows workstation.

The replication allows us to keep an updated copy of the centralized database in a remote station. This copy will not be used for local updates since it will be read-only. On the other hand, it allows us to work offline and much faster, since the exchanges between the central database and the local replicate contain only the updates of the centralized database. Continue reading “Replicating a postgresql database in a windows workstation”