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A refresher on vector and raster data structures with an emphasis on coordinate reference systems and spatial accuracy.
Why can a 1-meter error in your GIS data lead to a multi-million dollar construction disaster or a failed emergency response?
In professional GIS, we categorize the world into two primary models. Vector data represents the world as discrete objects using points, lines, and polygons. It is defined by topology, which describes the spatial relationships between features (e.g., 'this parcel shares a boundary with that road'). Conversely, Raster data represents the world as a continuous surface of grid cells or pixels. Each cell contains a value representing a phenomenon, such as elevation or temperature. The precision of a raster is determined by its spatial resolution, where the area of a single cell is .
Quick Check
If you are modeling the exact legal boundaries of a property, which data model provides the necessary discrete precision?
Answer
The Vector data model.
Earth is an irregularly shaped ellipsoid, but maps are flat. This transition requires a Coordinate Reference System (CRS). A Geographic Coordinate System (GCS) uses angular units (latitude/longitude), while a Projected Coordinate System (PCS) uses linear units (meters/feet). Every projection distorts at least one of four properties: Shape, Area, Distance, or Direction (S.A.D.D.). For example, the Mercator projection preserves direction but drastically inflates area as you move toward the poles.
To understand distortion, we look at the scale factor . On a perfect globe, . If a projection stretches a feature to twice its size, . 1. Identify the true distance on the ellipsoid (). 2. Measure the distance on the map (). 3. Calculate .
Quick Check
Why would a researcher use an 'Equal Area' projection when studying deforestation rates in the Amazon?
Answer
To ensure that the calculated size of the forest loss is mathematically accurate and not distorted by the map's projection.
Spatial accuracy is not assumed; it is verified. When georeferencing an old map or aligning satellite imagery, we use Ground Control Points (GCPs). The difference between the 'true' coordinates and the 'mapped' coordinates is the residual error. To determine the overall accuracy of a dataset, we calculate the Root Mean Square Error (RMSE). A lower RMSE indicates higher spatial integrity. In professional workflows, an RMSE exceeding a specific threshold (e.g., 0.5 meters for urban planning) requires the data to be re-aligned or discarded.
Which data structure is most efficient for performing a 'Slope Analysis' based on elevation changes?
If a GIS analyst finds an RMSE of for a high-precision utility map (required accuracy ), what is the correct action?
A Geographic Coordinate System (GCS) uses meters as its primary unit of measurement.
Review Tomorrow
In 24 hours, try to sketch the difference between a vector line and a raster line and explain how the RMSE formula relates to the Pythagorean theorem.
Practice Activity
Open a GIS viewer (like QGIS or ArcGIS Online) and compare the area of Greenland in 'Web Mercator' versus an 'Equirectangular' projection.