Coordinate Systems & Projections
The Problem: Rasters Displayed Incorrectly Rasters containing geographic data often need to be displayed with a specific orientation for visualization or analysis. However, rasters generated from various sources may not aligned properly when loaded into R. A common issue is a raster that spans the 180 degree longitude line having an inappropriate 360 degree orientation….
Understanding Coordinate Reference Systems A coordinate reference system (CRS) defines how latitude, longitude, and height positions are displayed on a map or model of the Earth. Defining the CRS requires setting the map projection, which represents the 3D earth surface on a 2D plane, and the datum, which models the size and shape of the…
The Problem of Mismatching Coordinate Reference Systems The QGIS clip tool allows users to clip or extract parts of a raster or vector dataset using features from another dataset as a clip boundary. This is a common geoprocessing workflow. However, the clip tool requires that the input raster and vector datasets have matching coordinate reference…
Purpose of Equal-Area Projections Equal-area projections aim to accurately represent the proportional areas of features on the Earth’s surface. By preserving the relative sizes of countries, lakes, and other geographical entities, equal-area projections facilitate analysis related to size and scale. Two key applications where preserving accurate areas is critical are oceanography and marine biology. In…
Defining Precision and Accuracy Precision refers to the level of measurement and exactness of a value. It relates to the fineness of an operation or the degree to which repeated measurements show the same results. Accuracy refers to how close a measured value is to the true value. An example to illustrate the difference is…
The Dilemma of Projection Choice Selecting the optimal map projection involves navigating complex trade-offs between preserving accurate shapes, areas, distances, and directions. The earth’s surface is three-dimensional and spherical, yet maps depict the world on a two-dimensional plane. Flattening and projecting the globe introduces distortions that vary based on the projection used. Certain projections accurately…
Defining the Problem: Overlapping UTM Zones and Inaccurate Area Calculations When mapping offshore areas that span multiple Universal Transverse Mercator (UTM) zones, inaccuracies in area calculations can occur. This is due to the division of the earth’s surface into 60 transverse Mercator projection zones in the UTM coordinate system, each spanning 6° of longitude. Adjacent…
