How To Read A Map
Maps have great visual appeal. They're easy to understand and interpret. They make boring data look like extraordinary data. They help you interpret other types of documents. They are time stamped objective records of the past. They're also inexpensive and easy to get. With so many compelling reasons to use maps in your day to day work more often, read on to learn some basic map reading skills to help you use modern mapping more effectively.
Aerial and satellite imagery from film or digital sources are adjusted geometrically to a consistent scale. High resolution imagery, such as from aerial acquisition, allows you to zoom in on small details. Mid and low resolution imagery, like satellite imagery, gives an overview of a large area.
Maps made of points, lines, areas, and text. Vector mapping is often created by categorizing and generalizing features seen in aerial imagery. Vector mapping can be used to represent real features like roads and water bodies, or, intangible features such as property lines.
What Can You Learn About A Place?
Land Use Patterns
Pathways and barriers to movement, or whether a land use type is homogeneous over a large area or not can give a clear idea of the types of activities you could expect to find at a location. If the area is urban or rural, for example, would be clear from the density of ground features and road networks.
Distances between features, relative sizes and physical arrangement of features, and clusters or disbursement of features or events are examples of spatial relationships between people, objects, and places. Configuration and distances between isolines in the samples below indicate steepness, slope direction, peaks and valleys, and channels where surface water will collect.
Changes Over Time
Aerial imagery is ideal for capturing intricate details of a snapshot in time. Comparing imagery of different vintages can reveal the progression of how, when, and where changes to the landscape took place.
Combining Data To Learn more
Context, Clarity, and Details
Layering vector data over a raster image can show where different geographic conditions intersect to help answer more complex questions. Is the house too close to the property line? Where will rain water flow in relation to the houses?
|Layering Vector Data Over Raster Data|
|These homes have a steep slope in the back yard.|
Geo-Data Interpretation Skills
Visual interpretation will quickly tell you something about the area you're looking at. Key factors to consider include patterns of land use and spatial distribution of ground features, the presence or absence of features, and changes over time. Raster data gives you the benefit of photographic details.
|The complex branching pattern tells us this map is a watershed.|
|Which direction is the current coming from?|
|The orderly arrangement of umbrellas and boats tells us this is an urban parkette.|
Maps and imagery can also be assessed quantitatively, for example, measuring distances and areas, or counting features. Counting objects on the ground in aerial imagery is a common method to make estimates when a census or other very precise methods are not available.
|Estimating the harvest yield expected from 10 hectares of land.|
|Counting cars in the parking lot as a proxy for economic activity.|
|What percent of forest is mature softwood species?|
Data About Data
Metadata includes information about the sources used to compile a map, the year it was published, the organization that produced it, technical specifications describing the scale, map projection, image resolution, and general keywords for searchability. Metadata can help you decide if the map will suit the purpose you want to use it for.
|Reviewing metadata on MapWarehouse before purchase.|
Scale and Resolution
Scale and resolution describe how sizes of features on the map relate to the real objects they represent. Vector maps are drawn to a scale, such as "1:10,000" meaning 1cm on the map equals 10,000cm on the ground. Raster maps use image resolution, such as "10cm" meaning each pixel covers 10cm x 10cm on the ground. Large scale vector maps and high resolution imagery can offer the most detail.
|Low resolution imagery|
|High resolution imagery of the same area|
In these vector maps of the same area, notice how the large scale maps outlines the footprints of the buildings in the NW corner, and the small scale map does not show this level of detail.
|Small scale vector map (1:25,000)|
|Large scale vector map (1:10,000)|
Selecting The Right Map
It depends what you're planning to do with it. Small scale or scantly detailed mapping is preferred for showing an overview of an area or where you don't want to distract the viewers' attention from one feature of interest. Large scale mapping and high resolution imagery is preferred where detailed context of a small area is needed.
Land appears to be vacant and uniform with small scale vector mapping.
High resolution imagery of the same area reveals complex land use patterns.
It is important to note that in these examples, all the mapping is correct and accurate. Depending on what conclusions you want the map audience to draw, reviewing metadata can help you choose the best map to support the point you want to make.
Less detailed elevation information gives the impression the land is flat.
The same area with more detailed elevation information shows topographic variability.
Data Processing and Manipulation
Vector data is normally created by tracing ground features seen in raster data, a process called digitizing that can be performed by visual inspection or using algorithms to identify subtle changes in colour to locate boundaries of objects. Vector data can only be as detailed as the source it was created from. The detail of the source photo is simplified according to the decisions of the cartographer. Care must be taken to ensure the maps you chose are showing the types of features you're interested in since vector maps can have a narrow focus, only showing roads for example, and exclude most other details.
Orthogonal, meaning at a right angle, is the objective in orthorectifying aerial imagery to ensure the map user has a flat, directly overhead perspective when viewing any section of the photo. Orthorectification is necessary to correct for movement and pitch of the aircraft, and to correct for the radial perspective of the camera which can make objects at the outside edge of the field of view appear to be leaning away from the viewer. Orthophoto is often made of a mosaic of several images taken from the aircraft a few seconds apart.
|Shape, distance, and direction are true in the overhead perspective, therefore this area measurement will be correct.|
|Notice the parallel sides of the building appear to be converging because of the oblique viewing angle. Distance and direction measurements will therefore also be distorted.|
An interesting feature of aerial imagery is that images taken a few seconds apart from an aircraft offer a slightly different viewing angle, similar to the different viewing angles from your right eye and left eye. Sequential pairs of images can be viewed together with the help of special glasses to create a 3D image that can be used to create elevation models, measure heights of trees and buildings, and find sight lines in three dimensions.
|Photogrammetry is the science of measuring aerial imagery.|
Related: Law Of the Land; Use mapping and aerial imagery to enhance your legal research
Where Can I Buy Geo-Data, Aerial Imagery, and Maps?
Ontario’s Largest Geospatial Data Marketplace
Search for data, select files covering your location, download your order, and work with aerial imagery and geographic data. Use the interactive map to locate your area of interest and purchase multiple data sets easily and quickly with a credit card.
VuMAP - Our Web Application
Access Every Place From One Place
View geographic data, query the map for more information, measure ground features you see in the imagery, and report on your research using this feature-packed online mapping application. Compare imagery year by year and research land information from anywhere with just and internet connection.
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Connect to a high resolution aerial imagery mosaic base layer and elevation contours served directly to your CAD or GIS workstation, just as you would connect to data stored locally. Bandwidth based pricing allows your team to use MapCast as little or as much as you need.