The second lab of the course focused on coordinate systems and spatial data modeling.
The learning objectives were as follows:
- Manage and preview data:
- Gain familiarity with the interfaces of ArcMap and ArcCatalog – the two basic applications used throughout this GIS course.
- Learn how to preview, manage, and explore the properties of spatial data.
- Understand coordinate systems and projections:
- Learn the proper procedures for creating, saving, and storing a map document properly.
- Understand the importance of reference systems in spatial data.
- Learn how to manage and change projections.
- Learn how to repair misaligned and improperly referenced spatial data.
- Learn the effect that various projections have on geometric properties.
- Establish best practices when working with spatial data and reference systems.
- Compare and contrast spatial data models:
- Understand the differences that exist between vector and raster data models.
- Learn the effect that data models have on the visualization and processing of spatial data sets.
- Learn how to add fields to an attribute table and how to perform simple calculations.
- Work with remote sensing imagery:
- Gain an understanding of how remote sensing operates.
- Learn to display remote sensing images.
- Learn to create a composite image using the different bands from satellite data.
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Misaligned and/or improperly referenced spatial data:
When dealing with geospatial data that is obtained from different sources, problem can arise with misalignment and/or improperly referenced data.
Projections are always based on a geographic coordinate system, and are a reference to a flat surface. Map projections transfer the spherical Earth onto a two-dimensional surface, effectively approximating the true shape of the Earth, presenting error in the spatial data and leading to distortion of some aspects, depending on which of the many projections is used. The four basic geometrical properties of distance, shape, direction, and area cannot all be held true simultaneously on a map projection, which leads to some of these properties being held true, while others are distorted. If a map is put together with layers using different projections, subtle differences can exist in terms of their alignment and reference coordinates, which can cause inaccuracy. Although misaligned or improperly referenced spatial data can occur, it can also be prevented through the use of a Geographic Information System, such as ArcGIS.
It is important that the projection of the various data-frames match the projections used for the area of the study, which can be checked by examining the properties of the various files, where information on coordinate systems, projects, datum, and units of measurement are available. In ArcMap, a function called “projection-on-the-fly” allows the user to combine layers with different coordinate systems, and automatically aligns them with each other without changing the spatial coordinates during the process. Another option is to actually modify the data by creating a new version of the data layer with different coordinate systems, which is done through a series of Transformation commands.
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Advantages of remotely sensed Landsat data for geographic analysis:
Landsat is a remote sending program that, since 1972, has been continually scanning the earth and providing high quality imagery of its surface. It measures the response of objects and surfaces on earth, using sunlight as its energy source. While a total of eight Landsat satellites have been launched since 1972, there are only two still in operation: Landsat 7 and Landsat 8. These satellites can scan the same area of the earth every 16 days, and the images captured are made freely available to the public.
There are several advantages to using the high quality images provided by Landsat satellites, for example: the still photos taken of the entire globe, every day, in moderate resolution have been archived since 1972. Furthermore, Landsat data and imagery are available for free. And finally, Landsat imagery contains many layers of data, which allows users to manipulate the images (Fort Collins Science Center). For over 40 years, Landsat images have provided data for applications in fields such as agriculture, cartography, geology, forestry, regional planning, and education.
Here is an example of a Landsat satellite image used in this lab. It was taken over Mt. St. Helen’s in 2002, and shows the drastic changes to the landscape after the eruption in 1980.
Fort Collins Science Center. Landsat imagery: A unique resource. URL: https://www.fort.usgs.gov/landsat-study. Accessed: April 12, 2016