One of the most common ways the round world is displayed on a map is using the simplest projection we have:. However we often seem to forget that we are talking about a projection. Therefore the spatial reference for this projection is very often mis referenced as a spherical coordinate system like the following for EPSG However, the moment we project this map onto a flat screen or piece of paper, we projected the coordinate system using the simple formula aboveso how can the spatial reference of our map be the above one?
I found that in ArcMap you can manually define a projected coordinate system that renders the same map, but correctly identifies the coordinate system as being projected. This has been done for so long, that this is now common mal practice. So why is this a problem? However, Microsoft SQL Server can correctly handle spherical coordinates, and suddenly this becomes a major problem.
Below is a query on all the counties that follow the red line defined by two points. The blue polygons is the result returned.
The map claims that its spatial reference is EPSG The real problem here is not that the line is not really a curve I drew the line like that because I want the features along that latitude. However had the application known that this was not a spherical coordinate system, but a Plate Carree projection, it would have known that when converting from the flat coordinate system to the spherical coordinate system, it should ensure that my line follows the latitude.
But because the input line already claims to be in spherical units, the application can not know that it needs to do anything to it aside the fact that the application of course knows that this came from a flat screen map, but the business logic is of course separate from the UI.
If I then think that my data is in EPSG or any other geographic coordinate systemthe border will NOT be following the 49th latitude, but go along a great circle that cuts into Canada. So let us thank Microsoft for creating a database that handles spherical coordinates correctly, and for giving us major headaches when trying to handle these things correctly in a clean clear way Cancel reply to comment.
Silverlight and WPF code reuse pitfalls. One of the most common ways the r I'm here at the build conference, where it was announced that we finally got access to the Windows P Comments 4 - Vish. Hi Morten, So, isn't the problem here that the drawing tools graphics layer on the browser lets you draw bad features straight looking lines that are not geographically straight.
Thank You, Vish. The problem is not the tool. It was made to work on a flat screen drawing lines that are straight in the projection. Since you are drawing on a flat screen, you are drawing in a projection. A geographic coordinate system cannot be drawn on a screen without projecting it first. That's the whole point of the post. We would need 2. Hi Morten, But don't you think that when the user draws a line from the eastern US to the Western US along the same latitude, the line should be displayed as a curve on the map instead of a line?
It means "latitude and longitude coordinates on the WGS84 reference ellipsoid. The term WGS84 is sometimes used the same way, but also it can refer to the ellipsoid only. For example, you can have "meters northing and easting as measured upon the cylinder formed by projecting the WGS84 ellipsoid using a transverse mercator projection with a central meridian of degrees".
In any case: No difference, just like everyone else is saying, except in the very nitty gritty details of how they are used.
They are certainly not the same. Those who think these are same actually ignore the basic purpose of EPSG. This ensures that coordinates describe position unambiguously. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered.
Ask Question. Asked 9 years, 5 months ago. Active 6 months ago. Viewed k times. Alex B Alex B 1, 2 2 gold badges 12 12 silver badges 12 12 bronze badges. Active Oldest Votes. Dan S. As far as I can see the two are the same thing.
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What is new? Online Registry The current version of the Online Registry which includes all recent updates version 9. The primary EPSG Dataset is maintained in the online registryfrom which data may be accessed through a graphic user interface or through a service interface.
The online registry contains the most current data. The online registry supports anonymous guest access, but also permits the user to register for additional services, such as the export of the entire Dataset.
Uses Spherical Mercator projection. Set in by default in Map's crs option. Something can't be right here, most likely my understanding. Google Earth is in a Geographic coordinate system with the wgs84 datum. EPSG: Google Maps is in a projected coordinate system that is based on the wgs84 datum. EPSG The Open Street Map tiles and the WMS webservice, are in the projected coordinate system that is based on the wgs84 datum.
So if you are making a web map, which uses the tiles from Google Maps or tiles from the Open Street Map webservice, they will be in Sperical Mercator EPSG or srid: and hence your map has to have the same projection. I'll like to expand the point raised by mkennedy. This convention is used in many places, such as:.
One way to show people what the differences in projection mean in practice is to draw a long line in Google Earth. By "long line" I mean one that is visibly a Great Circle route.
Everything's fine in Google Earth. Zoom in on the middle of the line to see how far the midpoint is displaced. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered.
Asked 7 years, 2 months ago. Active 5 years, 7 months ago. Viewed k times.
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Uses simple Equirectangular projection. Could someone help me understand? Active Oldest Votes. There are a few things that you are mixing up. While Making a KML, you will always give the coordinates in geographic Lat-long, even though it might be showed on top of a web Mercator map.Overview of Projection Support.
Layer Coordinate Reference Systems. Project Coordinate Reference Systems.
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Coordinate Reference System Selector. Custom Coordinate Reference System. Choosing an appropriate reference system for your QGIS projects and data can be a complex task, but fortunately QGIS helps guide you through this choice, and makes working with different CRSs as transparent and accurate as possible. See section Custom Coordinate Reference System for information on managing your custom coordinate reference systems.
In order to correctly project data into a specific target CRS, either your data must contain information about its coordinate reference system or you will need to manually assign the correct CRS to the layer. This projection file has the same base name as the.
For example, alaska. In some cases this is not possible, e. Under the CRS for layers group, set the action to do when a new layer is created, or when a layer is loaded that has no CRS. One of:. Convenient when loading a lot of layers at once.
Such layers will be identifiable in the Layers panel by the icon next to them. Sometimes, accompanying metadata will describe the correct CRS for a layer, in other cases you will need to contact the original author of the data to determine the correct CRS to use. Use project CRS. And press OK. Note that changing the CRS in this setting does not alter the underlying data source in any way, rather it just changes how QGIS interprets the raw coordinates from the layer in the current QGIS project.
The project CRS determines how data is projected from its underlying raw coordinates to the flat map rendered within your QGIS map canvas.
This means that regardless of the underlying CRS of particular map layers in your project, they will always be automatically transformed into the common CRS defined for your project. Choosing an inappropriate CRS can cause your maps to look distorted, and poorly reflect the real-world relative sizes and positions of features. Usually, while working in smaller geographic areas, there will be a number of standard CRSs used within a particular country or administrative area.
By default, QGIS starts each new project using a global default projection. This choice will be saved for use in subsequent QGIS sessions.
World Geodetic System
It will also be shown in the lower-right of the QGIS status bar. It can be used to guess a layer CRS based on its raw coordinates or when using QGIS for non earth uses like role-playing game maps, building mapping or microscopic stuff. In this case:. No reprojection is done while rendering the layers: features are just drawn using their raw coordinates.
A preview of the CRS extent on earth is displayed to help you select the appropriate one. Layers added to the project are translated on-the-fly to this CRS in order to overlay them regardless their original CRS.
Use of units and ellipsoid setting are available and make sense and you can perform calculations accordingly. For instance, some CRSs define their coordinates in feet instead of meters, so setting your QGIS project to one of these CRSs will also set your project to measure using feet by default. In the Layers panel, right-click on the layer you want to pick the CRS. Map canvas extent, coordinates display are updated accordingly and all the layers in the project are on-the-fly translated to the new project CRS.
This dialog helps you assign a Coordinate Reference System to a project or a layer, provided a set of projection databases. Items in the dialog are:. Enter the EPSG code, the identifier or the name.Coordinate reference systems CRS are important because the geometric shapes in a GeoDataFrame are simply a collection of coordinates in an arbitrary space.
A map projection or a projected coordinate system is a systematic transformation of the latitudes and longitudes into a plain surface where units are quite commonly represented as meters instead of decimal degrees.
This transformation is used to represent the three dimensional earth on a flat, two dimensional map. As the CRS in different spatial datasets differ fairly often i. It is important that the layers have the same coordinate reference system as it makes it possible to analyze the spatial relationships between the layers, such as conduct a Point in Polygon -query. Choosing an appropriate projection for your map is not always straightforward because it depends on what you actually want to represent with your map, and what is the spatial scale of your data.
In fact, the projection you choose might even tell something about you:.
For those of you who want a bit more analytical approach for choosing the projection, you can get a good overview from georeference. Luckily, defining and changing projections is easy in Geopandas. In this tutorial we will see how to retrieve the coordinate reference system information from the data, and how to change it. Shapefile should always contain information about the coordinate reference system that is stored in.
When reading the data into GeoDataFrame with Geopandas this information is automatically stored into. As we can see, here, the crs is a Python dictionary with a key init that has a value epsg There is also another typical way of representing the coordinate reference system, namely storing that information in Proj4-string format we will come back to this later.
As we can see, the coordinate values of the Polygons indeed look like latitude and longitude values, so everything seems to be in order as in most cases. Changing the projection is simple to do in Geopandas with. The function has two alternative parameters 1 crs and 2 epgs that can be used to make the coordinate transformation and re-project the data into the CRS that you want to use.
And here we go, the coordinate values in the geometries have changed! Now we have successfully changed the projection of our layer into a new one, i. To really understand what is going on, it is good to explore our data visually. Indeed, the maps look quite different, and the re-projected one looks much better in Europe as the areas especially in the north are more realistic and not so stretched as in WGS If this happens it is possible to fix the prj by passing the coordinate reference information as proj4-string.
For this purpose a library called PyCRS is a handy tool that makes passing the proj4-strings easy. PyCRS has functions pycrs. Our aim is to find the Euclidean distances from the centroids midpoints of all European countries to Helsinki, Finland.
We will calculate the distance between Helsinki and other European countries using a metric projection Azimuthal Equidistant -projection that gives us the distance in meters.
Notice, that this projection is slightly less commonly used, but still useful to know. As we can see, it is possible to create a GeoDataFrame directly with one line of code. To conduct the transformation, we are going to utilize again a Proj4-string that we can obtain using another library concentrated for coordinate reference systems, called pyproj.
Okay, now we can see that the coordinates in geometry column are fairly large numbers as they represents the distance in meters from Helsinki to different directions. As we can see from the map, the projection is indeed centered to Helsinki as the 0-position of the x and y axis is located where Helsinki is positioned.
The data may be used, copied and distributed subject to the following conditions: Whilst every effort has been made to ensure the accuracy of the information contained in the EPSG Facilities, neither the IOGP nor any of its members past present or future warrants their accuracy or will, regardless of its or their negligence, assume liability for any foreseeable or unforeseeable use made thereof, which liability is hereby excluded.
Consequently, such use is at your own risk. The data may be included in any commercial package provided that any commerciality is based on value added by the provider and not on a value ascribed to the EPSG Dataset which is made available at no charge. Subsets of information may be extracted from the dataset.
Users are advised that coordinate reference system and coordinate transformation descriptions are incomplete unless all elements detailed as essential in IOGP Surveying and Positioning Guidance Note Annex A are included.
Essential elements should preferably be reproduced as described in the dataset. Modification of parameter values is permitted as described in the table below to allow change to the content of the information provided that numeric equivalence is achieved.
Numeric equivalence refers to the results of geodetic calculations in which the parameters are used, for example i conversion of ellipsoid defining parameters, or ii conversion of parameters between one and two standard parallel projection methods, or iii conversion of parameters between 7-parameter geocentric transformation methods.
Equivalent single geocentric transformation in which for each parameter the parameter values of the component steps have been summed. Change of units 8 NTv2 method grid file filename. Convert unit to another, for example from microradian to arc-second, using conversion factors obtained from the EPSG dataset Unit table.
Contact IOGP. Change of ellipsoid defining parameters. Change of projection method. Change of transformation method.
Concatenated transformation using geocentric methods Geocentric translations, Position Vector 7-parameter transformation, Coordinate Frame rotation. Change of units.
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