|
10 | 10 | "\n", |
11 | 11 | "A Spatial Reference System (SRS) is a mathematical construct that is essential to the discipline of Geography. It has two important roles:\n", |
12 | 12 | " 1. identify unequivocally and with precision the location of a spatial object;\n", |
13 | | - " 2. accuratelly portray spatial objects in maps.\n", |
| 13 | + " 2. accurately portray spatial objects in maps. \n", |
14 | 14 | "\n", |
15 | 15 | "In its most basic form a SRS is composed by two elements:\n", |
16 | 16 | " 1. *Datum* - a sphere or ellipsoid that approximates the shape of the Earth, positioned relative to the later. \n", |
17 | 17 | " 2. *Cartographic Projection* - a set of mathematical functions that translate locations in the surface of the *datum* into the Cartesian plane.\n", |
18 | 18 | " \n", |
19 | | - "The WGS84 is standard *datum* issued by the World Geodetic Survey that is in wide use today. Most GPS or GNSS recievers today report geographic coordinates (e.g. longitude and latitude) in reference to this *datum*. For global cartography the WGS84 is an obvious choice, but for local mapping a bespoke *datum* can be more suitable. Most national surveys have defined *data* that suit appropriatly their country or region.\n", |
| 19 | + "The WGS84 is standard *datum* issued by the World Geodetic Survey that is in wide use today. Most GPS or GNSS receivers today report geographic coordinates (e.g. longitude and latitude) in reference to this *datum*. For global cartography the WGS84 is an obvious choice, but for local mapping a bespoke *datum* can be more suitable. Most national surveys have defined *data* that suit appropriately their country or region.\n", |
20 | 20 | "\n", |
21 | 21 | "\n", |
22 | 22 | "*Sketch of geodesic data positioned relative to the Earth's surface. Source: [Wikipaedia](https://en.wikipedia.org/wiki/Geodetic_datum).* \n", |
23 | 23 | "\n", |
24 | | - "The Marinus of Tyre and Mercator projections are the most popular today, however, none of them is suited for either local or global cartography (appart from navigation applications in the case of Mercator). It is always useful to spend some time identifying the most appropriate projection for the work at hand. Cartagraphic projections introduce errors as they flaten the curved surface of the Earth onto a plane. A balance must be struck between the accuracy of areas, shapes and angles. For global mapping, projections like [Mollweide's Homolographic](https://en.wikipedia.org/wiki/Mollweide_projection), [Eckert IV](https://en.wikipedia.org/wiki/Eckert_IV_projection) or [Goode's Homolosine](https://en.wikipedia.org/wiki/Goode_homolosine_projection) present interesting compromises. For local mapping, the [Stereographic](https://en.wikipedia.org/wiki/Stereographic_projection#Applications_to_other_disciplines), [Lambert's Azimutal Equal-Area](https://en.wikipedia.org/wiki/Lambert_azimuthal_equal-area_projection) and [Gauss-Krüger](https://en.wikipedia.org/wiki/Transverse_Mercator_projection#Ellipsoidal_transverse_Mercator) are popular choices, but many more exist.\n", |
| 24 | + "The Marinus of Tyre and Mercator projections are the most popular today, however, none of them is suited for either local or global cartography (apart from navigation applications in the case of Mercator). It is always useful to spend some time identifying the most appropriate projection for the work at hand. Cartagraphic projections introduce errors as they flatten the curved surface of the Earth onto a plane. A balance must be struck between the accuracy of areas, shapes and angles. For global mapping, projections like [Mollweide's Homolographic](https://en.wikipedia.org/wiki/Mollweide_projection), [Eckert IV](https://en.wikipedia.org/wiki/Eckert_IV_projection) or [Goode's Homolosine](https://en.wikipedia.org/wiki/Goode_homolosine_projection) present interesting compromises. For local mapping, the [Stereographic](https://en.wikipedia.org/wiki/Stereographic_projection#Applications_to_other_disciplines), [Lambert's Azimutal Equal-Area](https://en.wikipedia.org/wiki/Lambert_azimuthal_equal-area_projection) and [Gauss-Krüger](https://en.wikipedia.org/wiki/Transverse_Mercator_projection#Ellipsoidal_transverse_Mercator) are popular choices, but many more exist.\n", |
25 | 25 | "\n", |
26 | 26 | "\n", |
27 | 27 | "*Visual examples of the transformations applied to the *datum* by simple map projections. Source: [Wikipaedia](https://en.wikipedia.org/wiki/Map_projection).*\n", |
|
54 | 54 | "cell_type": "markdown", |
55 | 55 | "metadata": {}, |
56 | 56 | "source": [ |
57 | | - "A more formal way to initialise a `SpatialReference` object is using an [OGC Well Know Text](https://www.opengeospatial.org/standards/wkt-crs) definition. This is far more verbose, but also more accurate. The following example iniatilises a new object with the geographic system based on the WGS84:" |
| 57 | + "A more formal way to initialise a `SpatialReference` object is using an [OGC Well Know Text](https://www.opengeospatial.org/standards/wkt-crs) definition. This is far more verbose, but also more accurate. The following example initialises a new object with the geographic system based on the WGS84:" |
58 | 58 | ] |
59 | 59 | }, |
60 | 60 | { |
|
87 | 87 | "source": [ |
88 | 88 | "Note that no projection was defined, this system is supposed to refer strictly to longitude and latitude. However, most GIS programmes interpret this SRS as including the Marinus of Tyre projection (beware of distance or area computations).\n", |
89 | 89 | "\n", |
90 | | - "The [European Petroleum Survey Group (EPSG)](http://wiki.gis.com/wiki/index.php/European_Petroleum_Survey_Group) was a scientific body supporting the Petroluem & Gas industry in Europe. It developed an [extensive database](http://www.epsg.org/) of parametres and complete SRS definitions, that helped the industry standardise its cartographic processes. The EPSG assigned a unique numerical identifier to each entry in its database, which became rather handy to quickly refer to a specific, well defined, SRS. Most FOSS4G support the EPSG identifiers as quick reference (some even enforce it). PROJ and GDAL are no exceptions, and therefore `SpatialReference` objects can too be initialised with an EPSG identifier:" |
| 90 | + "The [European Petroleum Survey Group (EPSG)](http://wiki.gis.com/wiki/index.php/European_Petroleum_Survey_Group) was a scientific body supporting the Petroluem & Gas industry in Europe. It developed an [extensive database](http://www.epsg.org/) of parameters and complete SRS definitions, that helped the industry standardise its cartographic processes. The EPSG assigned a unique numerical identifier to each entry in its database, which became rather handy to quickly refer to a specific, well defined, SRS. Most FOSS4G support the EPSG identifiers as quick reference (some even enforce it). PROJ and GDAL are no exceptions, and therefore `SpatialReference` objects can too be initialised with an EPSG identifier:" |
91 | 91 | ] |
92 | 92 | }, |
93 | 93 | { |
|
104 | 104 | "cell_type": "markdown", |
105 | 105 | "metadata": {}, |
106 | 106 | "source": [ |
107 | | - "There are other methods to initialise a `SpatialReference` object, but these are the most common. There are also conversive methods, that export the parameters of into different " |
| 107 | + "There are other methods to initialise a `SpatialReference` object, but these are the most common. There are also converse methods, that export a `SpatialReference` into different formats that are simple to read:" |
108 | 108 | ] |
109 | 109 | }, |
110 | 110 | { |
|
163 | 163 | "cell_type": "markdown", |
164 | 164 | "metadata": {}, |
165 | 165 | "source": [ |
166 | | - "Question: the central point of the European system is in Potsdam, which lies at a latitude of 52º N. Why are none of the coordinates output above negative?\n", |
| 166 | + "Question: the central point of the European system is in Potsdam, which lies at a latitude of 52º N. Why are none of the coordinates outputed above negative?\n", |
167 | 167 | "\n", |
168 | 168 | "### Working with geometries\n", |
169 | 169 | "\n", |
170 | | - "In most cases work evolves around existing spatial objects. Conviniently, the `Geometry` class in the `osgeo.ogr` module provides a method named `Transform` that takes as argument a `CoordinateTransformation` object. The segment below exemplifies its use:" |
| 170 | + "In most cases work evolves around existing spatial objects. Conveniently, the `Geometry` class in the `osgeo.ogr` module provides a method named `Transform` that takes as argument a `CoordinateTransformation` object. The segment below exemplifies its use:" |
171 | 171 | ] |
172 | 172 | }, |
173 | 173 | { |
|
196 | 196 | "source": [ |
197 | 197 | "### Practical example\n", |
198 | 198 | "\n", |
199 | | - "In many cases it is necessary to work with spatial data created by someone else. It is then important to clearly identify the SRS of such data to make sure it matches the SRS used for analysis.\n", |
| 199 | + "In many cases it is necessary to work with spatial data created by someone else. It is then important to clearly identify the SRS of such data to make sure it matches the SRS used in the analysis.\n", |
200 | 200 | "\n", |
201 | 201 | "The example below opens a dataset with the borders of Romania and inspects its SRS. This type of data is detailed in the [Vector Data](04-vector-data.ipynb) section." |
202 | 202 | ] |
|
248 | 248 | "cell_type": "markdown", |
249 | 249 | "metadata": {}, |
250 | 250 | "source": [ |
251 | | - "What if a different SRS is required for analysis? The `transformEurope` object is used again for convinient. With the `TransformPoints` method it is possible to transform a list of coordinate pairs. This method returns a list a tuples, that is converted to a set of lists with the `zip` method." |
| 251 | + "What if a different SRS is required for analysis? The `transformEurope` object is used again for convenience. With the `TransformPoints` method it is possible to transform a list of coordinate pairs. This method returns a list a tuples, that is converted to a set of lists with the `zip` method." |
252 | 252 | ] |
253 | 253 | }, |
254 | 254 | { |
|
295 | 295 | "source": [ |
296 | 296 | "## Conclusions\n", |
297 | 297 | "\n", |
298 | | - "- Always check the coordinate system of each dataset before using it. Be it in complex analysis or simply to plot.\n", |
| 298 | + "- Always check the coordinate system of each dataset before using it. Be it in complex analysis or simple mapping.\n", |
299 | 299 | "- Make sure all datasets needed are in the same SRS before combining them.\n", |
300 | | - "- Take same time to identify the best SRS for the study at hand:\n", |
| 300 | + "- Take some time to identify the best SRS for the study at hand:\n", |
301 | 301 | " - Local versus global datum;\n", |
302 | 302 | " - Projection distortion properties." |
303 | 303 | ] |
|
313 | 313 | ], |
314 | 314 | "metadata": { |
315 | 315 | "@webio": { |
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319 | 319 | "kernelspec": { |
320 | 320 | "display_name": "Python 3", |
|
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