Coordinate Systems & Spatial Data Models

Learning objectives

This post is about topics explored in the first GIS laboratory session, which had the following learning objectives:

1. Understanding geographic data:

• How to review the properties of data to be used.
• The difference between “geographic coordinate systems” and “projected coordinate systems” and the reasons for using them.
• The problems associated with using each type of coordinate system.

 Misalignment and/or improperly referenced data

One problem that frequently occurs when dealing with geospatial data obtained from different sources is misaligned and/or improperly referenced data. Different parts of the world have different official or commonly used projected coordinate systems (or “projections”). Each projection is normally best suited for use in a specific area because of the geography of the local context. For example, the “Albers equal-area conic projection” is a standard projection used by the provincial governments of British Columbia and Yukon . However, this also means that they bear subtle differences between one another in terms of how they align and reference coordinates, and may thus cause inaccuracy if data layers use different projections. These inaccuracies could potentially be disastrous to the outcome of construction projects, for example.

These misaligned and/or improperly referenced geospatial data can be fixed through the use of a Geographic Information System (GISystems). Examples of GISystems are “Quantum GIS”, “ArcGIS” and “TerraView”.

Here is an outline of what needs to be done to fix misalignment and/or improperly referenced geospatial data:

1. Check whether there is misalignment or improper referencing in the first place by viewing the properties of each of the datasets or its metadata. The projection used by each dataset should be noted down. If this information are not available, it should be obtained directly from the provider of the dataset.
2. Next, check for the official or commonly used projections in the area of study. All datasets should be standardized to one projection and those datasets that do not use the selected projection should have their projections changed through the use of the GISystem.

More detailed steps (in ArcGIS) are as follows:

1. Preview the data and examine their attributes by right-clicking each file and going to “Properties”. Key information to look out for would be the coordinate system, projects, datum and units of measurement. For tiffs (raster layers), this information can be retrieved by going to the “General” tab and scrolling down until the “Spatial Reference” information can be seen. For shapefiles (vector layers), this information can be retrieved by going to the “XY Coordinate System” tab and looking at the box under the “Current coordinate system”.
2. Check for the official or common projections used for the area of study. For datasets that have projections different from the official or common projections, take note of them as there is a need to change or fix them. If a dataset is lacking in coordinate system information, the required information may be found in its metadata which can be accessed by going to the “Description” tab; if it is still absent, then there is a need to contact the provider of the dataset for the missing information.
3. The projection of the data frame should match the official or common projections used for the area of the study. Check (and change if required) its projection by right-clicking the data frame > “Properties” > “Coordinate System” > “Projected Coordinate Systems” and then selecting the required projection.
4. For those datasets where their projections are different from the official or common projections or they are lacking in coordinate system information, there is a need to change their projection by going to the Catalog window on the right and right-clicking the datasets > “Properties” > “XY Coordinate System” and then selecting the required projection.

 Advantages of using remotely sensed Landsat data

Landsat is a programme launched by the National Aeronautics and Space Administration (NASA) of the United States Government to apply space technology to Earth mainly for the enhancement of environmental management (NASA, 2015). A total of eight Landsat Earth observation satellites have been launched since the start of the programme in 1972, with two (Landsat 7 and Landsat 8) currently still in operation. Still images are captured by Landsat satellites about 350 number of times every day (USGS, 2012), with the images captured by Landsat 7 being made freely available to the public since October 2008.

Although other Earth-observing satellites exist, there are several advantages of using Landsat images (FORT, 2015):

1. Landsat satellites take still photos of moderate resolution of the entire globe every day which have been archived since 1972, allowing for both global studies and longitudinal studies of up to 43 years and counting.
2. Landsat data is free for public use. Satellites are expensive to manufacture and operate; having remotely sensed imagery of such quality being made freely available to the global public is rare.
3. Landsat imagery “contain many layers of data collected at different points along the visible and invisible light spectrum”, allowing users to manipulate these images for detailed studies of Earth’s surfaces.

These advantages of Landsat images have benefited society by not only reducing the economic costs of environmental management but also enhanced academic research towards many disciplines related to the environment (climate change, agriculture, forestry, water, land-use and land cover change, natural disaster management, and wildfires) which would have improved government policies and decision-making (USGS, 2012).

References

[FORT] Fort Collins Science Center. Landsat imagery: A unique resource. Retrieved September 28, 2015 from https://www.fort.usgs.gov/landsat-study.

[NASA] National Aeronautics and Space Administration (September 24, 2015). Case studies: How Landsat helps us. Retrieved September 28, 2015 from http://landsat.gsfc.nasa.gov/?page_id=6724.

[USGS] United States Geological Society (2012). Benefits of open availability of Landsat data. Retrieved September 28, 2015 from http://www.unoosa.org/pdf/pres/stsc2012/2012ind-05E.pdf.