What is the difference between raster and vector data?
In a GIS, a raster is a regular grid of cells, while vector data represent coordinate-based points, lines and polygons. Satellite imagery and elevation often use rasters; a road centerline or administrative boundary is often stored as a vector feature. The formats can be converted, but conversion changes how detail, edges and area calculations behave.
Cell grids and geometries
1. What does a raster represent? Each raster cell holds a value or series of spectral-band values. The form suits sampled continuous properties such as elevation, temperature and reflectance. Cell size and alignment determine which small features can be resolved. A thirty-meter cell containing plants and part of a road may hold their combined signal. Missing data, commonly called NoData, differ from an observed zero. The number of cells grows rapidly as geographic coverage expands or pixel size shrinks, increasing storage and computation. Enlarging a coarse raster on a screen cannot reveal objects smaller than those measured by its source. 2. What does a vector represent? A point can locate a well, a line can represent a road center, and a polygon can outline a lake. Each feature can have attributes such as name, date or class. Network connections among lines suit route analysis; polygons suit intersection and boundary queries. A vector boundary is not automatically exact or timeless. A shore changes through seasons and every survey has uncertainty. Simplifying a curved line to fewer vertices changes its geometry. A single line representing a road does not record its physical width unless an additional attribute or geometry provides it. 3. Which questions suit each format? Spatially varying elevations or satellite spectral bands fit the raster model naturally. Junctions on a bus route and buildings with addresses often fit vector geometry. USGS notes that roads, streams and other features can be represented in either raster or vector form; no single format is compulsory. Choose according to scale, the original observation and the analysis being performed. A blocky looking raster may contain a carefully calibrated quantitative measurement, while a beautifully smooth vector line can still be based on inaccurate source material.
Conversions and measurement consequences
4. How is a vector converted to raster? Values are assigned to grid cells that meet a rule involving a point, line or polygon. Cell size determines whether a narrow creek disappears. The edge of a polygon becomes stepped, and choices such as whether a cell center lies inside change the estimated area. QGIS and GDAL conversion tools require target width or resolution to be set. When multiple features compete for one cell, a priority rule is needed. Counting occupied cell areas after rasterization need not equal the exact area of the original vector geometry. 5. How is a raster converted to vector? Adjacent cells of one category in a classified raster can be grouped into polygons. QGIS can create polygons following individual raster cell extents and skip NoData cells. The output edge is often stair-stepped. Smoothing it can make a map visually appealing but does not add measurement detail. A continuous elevation raster generally needs a contour level or threshold before it can become meaningful lines or polygons. Pixel error and resolution limitations persist after the export even if the resulting file is stored as vectors. 6. What should be checked when combining them? Match coordinate systems, pixel alignment, resolution and definitions of mapped features. A coastline polygon and a thirty-meter water pixel should not be expected to coincide exactly. Compute areas and lengths under an appropriate projection; do not confuse screen smoothness with accuracy. Share file format, date, NoData rules and conversion settings for reproducibility. Legal property or safety decisions demand appropriate official source precision regardless of whether a layer happens to be a raster or a vector.