New HEC-RAS v 5.0.4


A new version of HEC-RAS was released; HEC-RAS 5.0.4. The first question to HEC-RAS user's is What are the main improvements of this version?
Actually, there are several improvements. Moreover, considering the software versioning standards, I think it would have been better to call it version 5.1. I will not go into the detail of all the improvements (the release notes is a 19 pages document), but I will summarize the most important ones.

       Computational speed. Computational speed is crucial in selecting a 2D model. I remember my first experience with HEC-RAS 5 beta took several days to perform a simulation; that same simulation took just about 2 hours using other models. Fortunately, HEC-RAS has been improving its computational speed. This new version has a fully parallelized engine that doubles the computational speed. If your previous simulation required X hours, with this new version it will only require 0.5 X hours.
       Ras Mapper tools. This one is a major improvement. Previous version still required ArcGIS and GeoRAS for preparing geometric data, especially for 1D cross sections and 1D profiles. This new version includes full GIS geometric processing capabilities. This video shows the creation of a 1D river geometry with the new Ras Mapper.
       2D nested mesh. Previous version performed 2D simulations on a nearly regular non staggered grid. That means the 2D DX was the same for all the grid. The new version allows to refine the grid at specific locations. Hence, a coarse grid can be used for big floodplains, while a more refined grid can be used for particular areas requiring more detail.
       More sediment options. New linear scaling factors and granulometry options are available
       64 bit processor. Most users may not be deep into computer science and may neglect the importance of this improvement. Let me tell you that this is an important improvement with BIG benefits. It allows working with bigger data files and faster processing. You will easily realize this benefits when working with the 2D; you will see how the mouse movements and the clicks are faster and your work is more comfortable.

Image 1. Print screen from the video showing 1D creation with the new RasMapper

Nevertheless, there is one disadvantage that you have to consider before using this new version. Although this 5.0.4 version is compatible with previous versions, previous version are not compatible with models saved under 5.0.4 version.

As I you can see, this new version is not limited just to fixing bugs. It has important add-ons and improvements. Therefore, considering the software versioning standards, I consider that it could have been  more appropriate to call it version 5.1.

Web Mercator is different than Mercator projection



The use of mobile apps and online maps is increasing. Everybody has access to and uses online map services such as google maps, street view or ESRI online maps. However, very often when GIS professionals use such maps as base for geo-referencing data, they forget that it uses a different coordinate system. Besides, this is a new coordinate system that sometimes has different names; this led to some confusions.

This new reference system is the so-called Web Mercator or Pseudo Mercator or EPSG3857. Many people wonder what's the difference? and why a Feature class using a Mercator projection changes when transformed into Web Mercator?

What is the difference?

A first and simple difference is that they have different false Easting and false Northing data. However, that is not the main difference. A standard Mercator projection is cylindrical conformal projection. I stress the word conformal because it is a key feature that made this projection so popular and the big difference with the Web Mercator. A conformal projection as defined by J Snyder means that the shape of every small feature of the map is shown correctly, the relative angles at each point are correct, and the local scale in every direction around any one point is constant. Thus, it preserves shapes and angles. On the other hand, Web Mercator is not conformal.

Fig 1. Conceptual difference between spherical and ellipsoid projections

Why Web Mercator is not a conformal projection?

Although Web Mercator and Standard Mercator use the same equations, they are based on different shapes. Standard Mercator assumes an ellipsoid, while Web Mercator assumes a sphere. Thus, there are differences that should be corrected by a scale factor. However, including such correction would be computationally expensive. Web Mercator is used online and requires simple equations. Otherwise, the visualization would be slow. Thus, they neglected such corrections.

The differences are function of the latitude. Latitudes closer to the equator have small errors, and the error increases as the latitude distances from the equator. The shape error in most populated areas (less than 50 degrees latitude) are small and negligible for a general visualization purposes; the computational cost of correcting such differences would be too high for such little differences. However, if detailed measurements are required, the errors would become important.

Fig 2. Error of Web Mercator projection


Tropical storms: Hurricanes, Typhoon, Clycones


Last week the Caribean islands, America and Gulf of Mexico experienced 3 Hurricanes in a row. At the same time, important Typhoons are developing on the Asia-Pacific region.

Basically, the three terms are the same. They all are tropical storms caused by the same reasons. Moist over the warm tropical ocean rises causes a low pressure area. Surrounding air pushes into this lower pressure area; thus, this air warms and rises. As this warm air rises, it cools forming cliuds and rain. Because of the Coriolis force this whole system of air, cloud, rain and wind beging spinning and growing.

Fig1. Formation of a tropical storm

The only difference between Hurricanes, typhoons and Cyclones is the local name they receive in different regions. 

Fig 2. Tropical storms different names

Hurricane
Tropical storms in the American region (Atlantic Ocean and America-Pacific) are known as Hurricanes. The word originates from the Mayan storm god Hunraken. Mayan people and other people from the region used the word to describes the stroms that were though to be a punishment from the god Hunraken

Typhoon
Tropical storms in the Asia-Pacific region are called Typhoons. In 1560 the Portuguese sailor Pinto made the first description of this tropical storm by using the word tufao. Tufao derives from the Chinese words tung (east) fung (wind). Chinese used these words to describe the strong windy storms coming from the East.

Cyclone
Cyclones are tropical storms on the Indian ocean. The first reference to this word is from the British sailor Piddington. The word originates from the Greek words kyklon (moving in a circle).
Strongest Tropical storms

Classification
Tropical are classified into 5 categories based on their wind speeds.
Category Wind speed [mph]
1 74-95
2 96-110
3 111-129
4 130-156
5 >156

Strongest tropical storms

Storm Year Winds [km/h] Wind [mph] Location
Patricia 2015 345 214 South of Mexico
Allen 1980 305 189 Caribean, North of Mexico
Irma 2017 295 183 Caribean, Florida
Wilma 2005 295 183 Gulf of mexico
Linda 1997 295 183 South of Mexico