A general-purpose map, also known as a reference map, provides a broad overview of geographical features and is designed to give users a sense of the spatial context of an area. These maps serve as a foundational layer, often used as a Basemap in a GIS project to support more specific geographic data or thematic maps. By providing essential information like roads, water bodies, terrain, and place names, general-purpose maps help users understand the geographic framework within which other spatial information is analysed.
Common Uses of General-Purpose Maps
General-purpose maps are widely used in GIS and cartography as basemaps for various purposes:
- Setting Context: These maps provide the necessary spatial context for overlaying thematic data, such as population density, land use, or environmental conditions.
- Navigation and Location: In urban areas, they serve as a reference for navigating through cities, understanding infrastructure, and visualizing proximity between features.
- Complementing Thematic Maps: They serve as the background for more specific or detailed thematic maps that focus on particular variables, like flood risks or demographic data.
Types of General-Purpose Maps by Area Type
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Urban Areas:
- In densely populated areas, it is common to use OpenStreetMap (OSM) as the general-purpose map. OSM offers detailed information on streets, buildings, and services, making it ideal for urban navigation, infrastructure planning, and service location analysis.
- Google Maps and other proprietary maps also function in this role, often including business information, transportation networks, and real-time data, which enhance urban mapping.
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Rural Areas and Countryside:
- In more rural or natural settings, a Topographical maps is often a more appropriate general-purpose map. Topographic maps focus on elevation, natural landforms, water bodies, and terrain features, making them invaluable in areas where natural geography is more relevant than man-made structures.
- These maps are widely used for hiking, rural planning, and land management, where detailed terrain understanding is crucial.
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Small Areas or Large-Scale Maps:
- When working with small areas at a large scale, such as individual plots of land, it is common to use an orthophoto as a reference. While orthophotos (geometrically corrected aerial or satellite imagery) are not strictly considered maps, they provide an accurate visual representation of the landscape and built environment. This makes them particularly useful in fields like environmental monitoring, urban development, or detailed site analysis.
- Orthophotos allow users to see actual structures, vegetation, and natural features as they appear in the real world, providing a more realistic background than abstracted maps.
Choosing the Right Basemap
The choice of a general-purpose map or reference map depends on several factors:
- Scale: In large-scale maps (small geographic areas with high detail), orthophotos or detailed vector data from sources like OSM may be more appropriate. In small-scale maps (large geographic areas with less detail), topographical maps or simpler vector maps are more useful.
- Purpose: For navigation, urban planning, or infrastructure analysis, OSM or other urban-focused maps are typically better. For environmental analysis or rural planning, topographical maps are more relevant.
- Clarity and Precision: General-purpose maps are typically abstracted to some degree to make information clearer. However, when precise real-world representation is needed, orthophotos may provide more valuable information.
Orthophotos as a Special Case
While orthophotos are not traditional maps, they are frequently used as basemaps because they provide a photographic representation of the area. They combine the visual accuracy of aerial photography with the geometric accuracy of maps, making them especially useful for high-detail work where precise visualization is required. In situations like land surveying or urban development, an orthophoto may be the best way to visually represent the real-world environment.
Conclusion
General-purpose maps, whether they are detailed urban street maps like OpenStreetMap, topographical maps for rural areas, or orthophotos for precise visualization, play a critical role in providing the geographical context necessary for deeper geographic analysis. They serve as foundational layers in many GIS projects, helping users situate their data within a known and understandable spatial framework. Choosing the right general-purpose map depends on the specific needs of the project, including the area of interest, the scale, and the purpose of the analysis.