Common Errors and Basic Concepts

Published

November 19, 2022

#PERSONAL NOTES

COMMON ERRORS

  1. During importing geospatial data, the coordinate system of the source data was either missing (such as due to missing .proj for ESRI shapefile) or wrongly assigned during the importing process.

  2. Input feature class contains less than 30 features.

    >> produce low reliability results.

  3. The input field in categorical attribute field.

  4. Use WGS84, or even undefined.

    >> Should always use projection based coordinates.


CONCEPT NOTES FOR GEOCOMPUTATION WITH R

Following notes are quoted from Geocomputation with R

  1. simple feature geometries (basic) > sfg - objects > sfc - ‘list columns’ of class > sf - (full objects)

  2. sfg class :

    2.1 - represents the different simple feature geometry types in R : point, linestring, polygon (and their ‘multi’ equivalents, such as multipoints) or geometry collection.

    • A point: st_point()

    • A linestring: st_linestring()

    • A polygon: st_polygon()

    • A multipoint: st_multipoint()

    • A multilinestring: st_multilinestring()

    • A multipolygon: st_multipolygon()

    • A geometry collection: st_geometrycollection()

    2.2 - sfg objects can be created from three base R data types:

    • A numeric vector: a single point

    • A matrix: a set of points, where each row represents a point, a multipoint or linestring

    • A list: a collection of objects such as matrices, multilinestrings or geometry collections

    #further read-up is required

  3. sfc class :

    3.1 - sfc is a list of sfg objects, which is additionally able to contain information about the coordinate reference system in use. For instance, to combine two simple features into one object with two features, we can use the st_sfc() function. This is important since sfc represents the geometry column in sf data frames

    #further read up is required

  4. sf represent a set of data by combining the attributes (data.frame) with the simple feature geometry column (sfc). They are created with st_sf() .

    #further read-up is required

  5. In geospatial analytics, it is very common for us to transform the original data from geographic coordinate system ( 0-360 range of decimal degree) to projected coordinate system.

    >> geographic coordinate system is not appropriate if the analysis need to use distance or/and area measurements.

  6. Shape objects can be vector or raster data

    • sf objects for vector data

    • stars objects for raster data

      In tmap, a shape object needs to be defined with the function tm_shape(). When multiple shape objects are used, each has to be defined in a separate tm_shape() call.

      tm_shape(worldelevation) +
  tm_raster("worldelevation.tif", palette = terrain.colors(8)) +
tm_shape(worldvector) +
  tm_borders() +
tm_shape(worldcities) +
  tm_dots() +
  tm_text("name")

      three shapes: worldelevation which is a stars object containing an attribute called "worldelevation.tif", worldvector which is an sf object with country borders, and worldcities, which is an sf object that contains metropolitan areas of at least 20 million inhabitants.

      Each tm_shape() function call is succeeded by one or more layer functions.

      The order of the tm_shape() functions’ calls is crucial. The first tm_shape(), known as the main shape, is not only shown below the following shapes, but also sets the projection and extent of the whole map.

Tobler’s First law of Geography

” Everything is related to everything else, but near things are more related than distant things.”

Foundation of :

  • spatial dependence

  • spatial autocorrelation


CODE REFERENCES

EPSG code geodetic CRS datum ellipsoid area of use
3414 - 4326 SVY21 SVY21 WGS 84 (4326) Singapore - onshore and offshore


REFERENCES

  • wiki.gis.com. Datum (geodesy) - http://wiki.gis.com/wiki/index.php/Datum_(geodesy)
  • Chapter 4 Specifying Spatial Data - https://r-tmap.github.io/tmap-book/tmshape.html


QUARTO QUICK REFERENCES

  • HTML Theming - https://quarto.org/docs/output-formats/html-themes.html

  • HTML Code Blocks - https://quarto.org/docs/output-formats/html-code.html

  • HTML Options - https://quarto.org/docs/reference/formats/html.html

  • Article Layout - https://quarto.org/docs/authoring/article-layout.html

  • Tables - https://quarto.org/docs/authoring/tables.html

  • reStructuredText Options - https://quarto.org/docs/reference/formats/rst.html