Overview

As part of the Oak Ridges Moraine NATMAP program, a 32,000 km2, geo-referenced LANDSAT Thematic Mapper (TM) mosaic has been created (Kenny, 1998). The mosaic encompasses the Oak Ridges Moraine (ORM) and Greater Toronto Area (GTA) of southern Ontario (Fig. 1). The images selected for this composite were recorded in the spring (Fig. 1). In northern, temperate, predominately agricultural areas, such as southern Ontario, spring imagery is the most informative for surficial geology and hydrology studies. Agricultural areas are bare of crops, and deciduous trees lack foliage permitting better surface observations. An additional advantage of spring imagery is that the winter accumulation of snow and ice has thawed, resulting in maximum water storage and flow in both surface and groundwater systems. With surface hydrology networks at a maximum, ephemeral drainage systems and wetlands can be observed and mapped. As well observable surface soil moisture characteristics on bare agricultural fields can generally be related to surficial and subsurface soil textural properties. To further emphasize the drainage features in this composite the hydrography from the 1:50,000 scale NTS base maps has been overlain.

The Red/Green/Blue colour composite presented here is composed of LANDSAT TM bands 4 (near infrared), 3 (red) and 2 (green) respectively. This band combination is commonly referred to as a "Standard False Colour Composite". The inclusion of an infrared band with red and green visible bands, increases the observable image information, particularly for the discrimination of vegetation communities and identification of hydrographic features (Lillesand and Kieffer 1987). To properly interpret this composite an interpreter need only be aware of a few additional characteristics of infrared radiation and a few properties of this particular colour band assignment.

This digital mosaic is being used for several functions within the Oak Ridges Moraine NATMAP project. The mosaic allows: terrain and surficial geological "photo-interpretation", merging with other image products to derive more geologically informative enhancements, and use as an image backdrop within GIS analysis. The synoptic 1:200,000 scale mosaic presentation is part of a series of complementary 1:200,000 scale map products including; 1) a digital elevation model (Skinner & Moore, 1997), 2) a chromo-stereo enhanced digital elevation model (Kenny, 1997), 3) regional surficial geology (Sharpe et al., 1997), 4) bedrock topography (Brennand et al., 1998), 5) sediment thickness (Russell et al., 1997), and 6) spring locations (Dyke et al., 1996). The information presented in these maps and enhanced image products reveal the unique character, spatial definition and the controlling physical environmental influence the Oak Ridges Moraine exerts on the region.

For reference six figures from Kenny (1998) are presented that illustrate the atmosphere, hydrologic, vegetative and increasing anthropogenic signatures in the "false colour" composite. Each figure is accompanied by a brief summary. The selected images were acquired in early spring (Mid May). This Red/Green/Blue composite combines Landsat Thematic Mapper Bands 4 (near infrared), 3 (red) and 2 (green). This band combination is referred to as either a "Standard False Colour Composite" or "Colour Infrared Composite". These images can be used as a guide to interpreting images in the accompanying section Images - TM.

Cloud Cover

Optimal use of visible and infrared imagery for earth observation, depends on cloud free imagery. Clouds and haze produce strong reflectance in all Landsat bands, although somewhat less on the longer-wave, infrared bands. The images selected for this mosaic were generally cloud free with very little haze, with the exception of the area seen over Presqu'ile Provincial Park, just south of Brighton, on Lake Ontario. An advantage of "Standard False Colour" composites over visible band composites is that, by the elimination of the blue spectra and the inclusion of infrared spectra, the effect of atmospheric haze can be reduced, providing clearer, easier-to-interpret, imagery.

Water Bodies/Wetlands

This scene shows the southern portion of Lake Scugog and the marsh to the south. Water absorbs most incoming red and green radiation and virtually all infrared radiation. As a result, water bodies appear black (Lake Scugog) in the absence of atmospheric haze or clouds. This characteristic can also be partially observed in wetlands, when not covered by emergent aquatic vegetation (eg. early spring). The combined impact of the bands selected for this composite and the image acquisition dates, (early spring), enhances the expression of hydrographic features. Numerous ephemeral wetlands and streams can be seen across this composite, particularly in the poorly-drained, fine textured soil areas.

Deciduous and Coniferous Forests

This forested area occurs east of Rice Lake on the sandy Oak Ridges Moraine, at the headwaters of the Granaraska river (flowing south) and Baxter creek (flowing north). Identification of tree species in this composite is aided by the knowledge that infrared radiation is strongly reflected by healthy vegetation. The leaf-less deciduous stands (A), are seen in dark grey and green hues, a result of low infrared reflection. The coniferous forests (B), (largely spruce and pine plantations) are seen in dark red hues, a result of high infrared reflection. Where as these forest stands are relatively homogeneous, many forested stands, including many riparian zones, are composed of mixed forest species (C) and have mixed forest spectral signatures. Forest tracks help maintain soil moisture in permeable sand soils of the Oak Ridges Moraine.

Agricultural Lands

The dominate landcover class across this map region is agricultural. The largest natural breaks in the agricultural areas are the numerous forested/wetland riparian zones, the Oak Ridges Moraine and the Niagara Escarpment. Agricultural areas are easily distinguished by characteristic rectangular field patterns and near mono-tone colours. The bright red areas (J) in the image are fields with sprouting crops. The light blue/grey tone fields (K) are cultivated fields that have no crops at the time of image acquisition. Generally, darker fields are wetter, as incoming radiation is absorbed by water. Dormant/abandoned agricultural and pasture lands (L) that are regenerating naturally do not produce the relatively uniform red tones as observed in the mono-cultural, cropped areas. The highest concentration of dormant and abandoned agricultural lands are seen in the highest elevation areas across the Oak Ridges Moraine. The sandy soils in this area are of marginal agricultural value due to low moisture-holding capacity.

Aggregate Pits

This area on the ridge of the sandy and gravelly Oak Ridges Moraine, near Musselman's Lake (ML), is on the drainage divide between Reesor/Little Rouge creeks flowing to the south and the Black creek flowing to the north. The irregularly shaped bright white areas (D), some with ponded water, are aggregate pits. All three bands in this composite have a strong reflectance on unvegetated, moisture-deficient, sand and gravel surfaces. The impact of aggregate mining on this landscape can be seen across the Oak Ridges Moraine as well as in selected rocky-gravelly areas along the Niagara Escarpment to the east. This same low-moisture spectral expression can also be seen in areas unassociated with aggregate pits, notably gravel concession roads, construction sites (see the Metropolitan Toronto urban fringe), and sandy beaches (see Wasaga beach on Georgian Bay).

Urban/Industrial

This urban zone is in west Mississauga, where highways 403 and the QEW intersect. Urban areas are identified by their mixed spectral signatures and a general lack of vegetation, and a low infrared reflectance (see downtown Toronto). Additional clues to urban/industrial areas are the regular patterns of side streets and roads. Cities and towns are however not entirely devoid of vegetation as can be seen by the presence of scattered bright red areas, which are parks (E), cemeteries and golf courses (F). Additionally, older subdivisions (G) with green lawns and tree-lined streets can be distinguished from newer subdivisions (H) by a somewhat stronger reflectance in the infrared band (i.e. redder). Urban and industrial construction sites (I) can also be identified by their bright reflection in all bands where soils are dry and lack vegetation (i.e. near white). The effect of recent urban growth on landscape can be seen across the GTA, particularly in the periphery around existing population centres.

References

Brennand, T.A., Moore, A., Logan, C., Kenny, F.M., Russell, H.A.J., Sharpe, D.R., and Barnett, P.J., 1997. Bedrock Topography of the Greater Toronto and Oak Ridges Moraine Areas, southern Ontario; Geological Survey of Canada Open File 3419, scale 1:200,000.

Dyke, L., Sharpe, D. R., Ross, I., and Hinton., M. 1996. Potential Springs from Aerial Thermography - Oak Ridges Moraine, Southern Ontario. Geological Survey of Canada and Ontario Ministry of Natural Resources, Geological Survey of Canada Open File 3374, scale 1:200,000.

Kenny, F.M. 1997. A Chromostereo Enhanced Digital Elevation Model of the Oak Ridges Moraine Area, Southern Ontario; Geological Survey of Canada and Ontario Ministry of Natural Resources, Geological Survey of Canada Open File 3423, scale 1:200,000

Kenny F.M. 1998. A Landsat Thematic Mapper Mosaic of the Oak Ridges Moraine and the Greater Toronto Area of Southern Ontario; Geological Survey of Canada and Ontario Ministry of Natural Resources, Geological Survey of Canada, Open File 3646, scale 1:250 000

Lillesand, T.M., and Kieffer, R.W., 1987, Remote Sensing and Image Interpretation, Second Edition. New York, John Wiley and Sons. 721 p.

Russell, H.A.J., Moore, A., Logan, C., Kenny, F.M., Brennand, T.A., Sharpe, D.R., and Barnett, P.J., 1997. Sediment Thickness of the Greater Toronto and Oak Ridges Moraine Areas, southern Ontario; Geological Survey of Canada Open File 2892, scale 1:200,000.

Sharpe, D. R., Barnett, P.J., Brennand, T.A., Finley, D., Gorrell, G., Russell, H.A.J., and Stacey, P., 1997. Surficial Geology of the Greater Toronto and Oak Ridges Moraine Area, southern Ontario; Geological Survey of Canada Open File 3062, scale 1:200,000.

Skinner, H., and Moore, A. 1997. Digital Elevation Model of the Oak Ridges Moraine, southern Ontario (hillshade enhanced). Geological Survey of Canada and Ontario Ministry of Natural Resources. Geological Survey of Canada Open File 3297, scale 1:200,000.