Modern Mapping: GIS, GPS, and Remote Sensing

Modern mapping uses technology to collect, store and display geographic data: a Geographic Information System (GIS) stores and analyzes spatial data in layers, the Global Positioning System (GPS) finds a position from satellite signals, and remote sensing gathers information about Earth from a distance using satellites and aircraft. In brief: GPS answers “where am I?”, remote sensing answers “what is on the surface and how is it changing?”, and GIS combines the data in layers to answer questions such as “which sites are within 500 m of a river and on gentle slopes?”. Together they have largely replaced paper surveying and drawing.

These tools are used for navigation, planning, farming, disaster response and environmental monitoring. This guide explains GIS, satellite imagery, remote sensing and GPS, compares them in a table, works through numerical examples (signal travel time, image resolution and a vegetation index), outlines the history and corrects common mistakes.

What is GIS?

A Geographic Information System is a computer system for capturing, storing, analyzing and displaying data linked to locations. Information is organized in layers, each one a theme (roads, rivers, land use, elevation, population) in the same coordinate system, which can be switched on and off and combined. Data are of two types: vector (points, lines and polygons, such as wells, roads and parcels) and raster (a grid of cells with values, such as elevation or satellite images). A table of attributes is linked to each feature. GIS analysis includes buffering (zones at a set distance from a feature), overlay (combining layers to find where conditions coincide), network analysis (shortest routes) and creating maps. Uses include planning cities, choosing sites for shops or wind farms, tracking disease, managing emergencies and conservation.

What are satellite imagery and remote sensing?

Remote sensing is the collection of data about the surface without touching it, from sensors on satellites, aircraft and drones. Most sensors record electromagnetic radiation reflected or emitted by the surface. Passive sensors measure sunlight reflected in visible and infrared bands, or heat emitted; active sensors, such as radar and lidar, send their own signal and record the echo, and so can work at night and through clouds (radar). Different surfaces reflect differently: healthy vegetation reflects a lot of near-infrared light and absorbs red, water absorbs most infrared, and bare soil and built-up areas have their own patterns. The main properties of an image are its spatial resolution (the size of the ground area per pixel), its spectral resolution (the number of bands), its temporal resolution (how often the same place is imaged) and its coverage. Applications include mapping land cover, monitoring deforestation, floods and urban growth, estimating crop health and measuring sea ice.

How does GPS work?

GPS is one of the Global Navigation Satellite Systems (GNSS); others include GLONASS, Galileo and BeiDou. The GPS constellation has at least 24 satellites in orbits about 20,200 km above Earth, each one carrying an atomic clock and broadcasting its position and the time. A receiver measures the time delay of the signal from each satellite and multiplies it by the speed of light to get the distance to it. A distance from one satellite places the receiver on a sphere; the intersections of spheres from three satellites give a position, and a fourth satellite is needed to correct the error in the receiver’s clock, so at least four satellites are used for a three-dimensional position and time. Ordinary phone receivers reach an accuracy of a few meters; survey-grade systems with corrections reach centimeters. Errors come from signal delays in the atmosphere, reflections from buildings, and poor satellite geometry.

The technologies compared

Worked example: signals, pixels and vegetation

1. GPS distance. A signal takes 0.070 s to travel from a satellite to a receiver. The speed of light is 299,792 km/s, so the distance is 0.070 × 299,792 = 20,985 km, which is of the right order for a satellite at an altitude of 20,200 km that is not directly overhead.
2. Why timing precision matters. A clock error of 1 microsecond (10⁻⁶ s) causes a distance error of 299,792 km/s × 10⁻⁶ s = 0.3 km = about 300 m, so satellites carry atomic clocks and the receiver solves for its own clock error with the fourth satellite.
3. Image resolution. A satellite image has a pixel size of 30 m. Each pixel covers 30 × 30 = 900 m². An area of 10 km × 10 km (100 km² = 100,000,000 m²) contains 100,000,000 ÷ 900 = about 111,000 pixels. A house of 10 m × 10 m is smaller than one pixel and cannot be seen as a separate object.
4. Vegetation index. The normalized difference vegetation index is NDVI = (NIR − Red) ÷ (NIR + Red). A pixel of dense crop has a reflectance of 0.50 in near-infrared and 0.10 in red: NDVI = (0.50 − 0.10) ÷ (0.50 + 0.10) = 0.40 ÷ 0.60 = 0.67 (high, healthy vegetation). A pixel of bare soil with NIR 0.30 and red 0.25 has NDVI = 0.05 ÷ 0.55 = 0.09 (low).
5. GIS overlay. To find sites for a new campsite, a GIS selects the polygons that are less than 500 m from a river (buffer layer), on a slope of less than 5° (derived from a raster elevation layer) and not within a protected area; the overlay shows the cells that meet all three conditions.

How did modern mapping develop?

Aerial photography for mapping began in the early 20th century and was used widely in the First and Second World Wars. The first Earth observation satellite programs started in the 1960s, and the Landsat series, launched in 1972, has provided a continuous record of images ever since. Roger Tomlinson developed the Canada Geographic Information System in the 1960s, and he is often called the “father of GIS”. GPS was developed by the United States military from the 1970s, the full constellation was declared operational in 1995, and civilian access was improved in 2000. Today smartphones, drones and web maps put these tools in the hands of everyone.

Which mistakes are common?

Mistake 1: Treating GIS as just a program for making maps

Its real strength is analysis: combining layers and asking questions about the data.

Mistake 2: Thinking that GPS needs only three satellites

Three give a position if the clock is perfect, but receivers need a fourth to solve for their clock error.

Mistake 3: Confusing resolution with image quality

A smaller pixel size gives more detail, but a high-resolution image covers a smaller area and may be taken less often.

Mistake 4: Trusting the output without checking the data

Maps and analyses are only as good as the quality, date and scale of the input data. Check them on the ground (“ground truthing”) where possible.