RAS Mapper has allowed hydraulic modelers to more easily perform Geographic Information System (GIS) operations. Many people think that the RAS Mapper and the Geometric Data Editor will eventually merge into one interface. If you want to use this increasingly popular feature in HEC-RAS (RAS Mapper), you must add a projection to your HEC-RAS project.
An ESRI projection file (.prj file extension) contains information about a particular coordinate system. A projection file is used in GIS applications to georeference vector data. In HEC-RAS, an ESRI projection file is required to access the base maps available in RAS Mapper.
This blog post will outline each component of the ESRI projection file and where to find projection files. First, I will briefly describe a few common coordinate systems. This will help you develop a more solid understanding of the information listed in the ESRI projection file.
Coordinate Systems
A coordinate system is used in Geographic Information System (GIS) applications as a framework to represent locations. There are many coordinate systems used in GIS applications. They can be classified into two broad categories: geographic coordinate systems and projected coordinate systems.
Geographic Coordinate System
A geographic coordinate system represents the Earth using a three-dimensional (3D) spherical surface. Locations on Earth are represented by coordinates (latitude and longitude). Latitude and longitude are angles measured from the center of the Earth to a point on the Earth’s surface.
Geographic coordinate systems are also called global or spherical coordinate systems.
Projected Coordinate System
In contrast to a geographic coordinate system, a projected coordinate system is based on a map projection. A map projection flattens the Earth’s surface into the plane. Because there is no perfect way to convert a curved surface to a flat surface, there are many projected coordinate systems available.
Depending on the type of map projection used, different spatial properties will be distorted. Some of these properties include distance, area, shape, and direction.
Some of the most common types of map projects are discussed below.
Equal Area Projections
An equal-area projection is designed to maintain the area of specific features. In order to do this, these projections distort shape, angle, and scale. The Albers Equal Area Conic projection is an example of this type of map projection.
Conformal Projections
Conformal projections maintain the shape of a small area. However, this means that the size of features is distorted. The Univeral Transverse Mercator (UTM) projection and the Lambert Conformal Conic projection are examples of conformal projections.
Equidistant Projections
An equidistant projection maintains the scene along one or more lines. The distance between the center of the map and any other point is correct in an equidistant projection. This type of projection is useful for point-to-point communication (telecommunication) applications.
Azimuthal Projections
Azimuthal projections plots preserve the direction from one point to all others. This is done by projecting the Earth’s surface onto a flat plane. Azimuthal projections can be combined with equal area, conformal, and equidistant projections. Some common azimuthal projections include gnomonic, stereographic, and orthographic projections.
| Azimuthal Projection | Description |
| Gnomonic | A great circle is a circle on the globe such that the plane passing through the sphere’s center is equal to the circumference of the Earth. For this projection, each great circle is mapped to a straight line. Seismologists frequently use this projection because seismic waves travel along with circles. It is also a projection that is used for navigation. This is one of the oldest map projections used today. The gnomonic projection is not equal-area or equidistant. These two properties increase as you move away from the center point. |
| Stereographic | This projection maintains direction from the center point. Each line represents a great circle. The stereographic projection is not equal area or equidistant. |
| Orthographic | The orthographic projection involves projecting the globe onto a plane with the point of projection as infinity. As indicated in the name, all projection lines are orthogonal to the projection plane. This is a map projection commonly used for inset maps. |
ESRI .PRJ File
The ESRI projection file has a *.prj file extension. It is formatted in a specific way to convey information related to a particular coordinate system. A projection file is comprised of a single string of text written in the “Well-Known Text” (WTK) format. The image below is the projection file for the Muncie example project that comes with HEC-RAS.
The entire projection file is wrapped in the PROJS[], which stands for the projected coordinate system. The PROJS[] clause begins with the official name of the projected coordinate system. In the example above, this is “NAD_1983_StatePlane_Indiana_East_FIPS_1301_Feet.” Although this may seem like a random name, each coordinate system has a specific name to use in this part of the PROJS clause. After this statement, there are more sub-clauses that define various aspects of the projection. These are summarized below.
| Sub-Clause | Description | Notes |
| GEOGCS[] | Geographic Coordinate System | This sub-clause defines the geographic coordinate system. This sub-clause contains additional sub-clauses (DATUM, PRIMEM, and UNIT). |
| DATUM[] | Datum | This sub-clause defines the horizontal reference datum. In the example above, this sub-clause contains another sub-clause that defines the shape of the earth, SPHEROID. |
| PRIMEM[] | Prime Meridian | This sub-clause defines the reference meridian used for longitude measurements. |
| PROJECTION[] | Projection | This sub-clause defines the projected coordinate system used to convert geographic coordinates to projected coordinates. |
| PARAMETER[] | Parameter | Projection files consist of several PARAMETER[] sub-clauses that define the false northing, false easting, the latitude of origin, scale factor, and central meridian. |
| UNIT[] | Units | This sub-clause defines the units used to measure distance. In the example above, you will notice that the UNITS sub-clause is different for the geographic coordinate system and the projected coordinate system. |
It is important to note that the ESRI projection file and HEC-RAS project file have the same file extension. Both of these files will (probably) be located within your HEC-RAS project folder, so do not confuse them. Selecting the wrong file will not mess up your model as RAS Mapper will only accept properly formatted projection files.
Where To Find Projection Files
You can use the projection file from a shapefile to define the projection in HEC-RAS. Alternatively, you can obtain projection files from spatialreference.org. After searching for a specific projection file, you will end up on a screen that looks like the one below.
How To Import A Projection File Into HEC-RAS
After you have obtained a projection file, you must import it into HEC-RAS. To import a projection file, complete the following steps.
- Open RAS Mapper.
2. Click Tools > Set Projection for Project
3. Navigate to the folder containing the ESRI projection file.
Troubleshooting
If your projection file produces the following error in RAS Mapper: “Error – unable to make a projection with the specified file. Make sure specified file is a valid ESRI projection (*.prj) file,” open the projection file in Notepad and try following:
- Check that all information looks correct. Some projection files available on the internet will only contain the GEOGCS section
- Remove any vertical datum statements (begins with VERTCS)
- If you edit your projection file and browse for the same file in RAS Mapper, the program will not read it again. Browse for any other projection file, click Ok, and then go back to the correct projection file.
If your project ends up in the wrong place (like the middle of the ocean), make sure the projection file’s units (e.g., feet or meters) match the HEC-RAS project.
In addition, you should make sure that the projection file is consistent with the map projection associated with the cut lines defined in the geometric data editor.