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Slope and Aspect

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What Are Slope and Aspect? 

Slope, the steepness of a surface, is a concept we’re familiar with in day-to-day life but has more serious implications for the process of land development. For technical use, the slope is expressed either as a percentage or as an angle degree. In simple mathematical terms, slope can be calculated in percentage with the formula “Rise over run”: rise/run = % slope, rise being the vertical height of the land at its highest point and run being the horizontal length of the area of land. To calculate the degree of the slope, input arctan(rise/run).

Aspect is the directional orientation of the slope. It is measured in degrees out of a clockwise 360-degree circle, with 0 degrees representing North, 90 indicating East, and so on in increments of 90 degrees in a clockwise direction, with each 90-degree increment indicating a cardinal direction.  

 

Why Are Slope and Aspect important to Land Developers?

For any land being considered for development, Geographic Information Systems (GIS) is used to map and measure the physical features of the land, including its slope and aspect, in order to make the best decisions in terms of land use, future architecture, and mitigate environmental impact. GIS experts use a variety of tools, including 3-D mapping and modeling. Digital Elevation Models (DEMs) are digital graphics used to depict the topography of the land using small pixels or cells called rasters. Slope can be measured from DEMs by counting the number of rasters of the rise and run of a certain area. Remote sensing is also an important information-gathering mechanism for GIS professionals. Common remote sensing technologies include drones, airplanes, helicopters, and Low Earth Orbit satellites. 

The slope of the land being considered for development determines the very foundation of construction on the site. In some places, local legislation requires mitigation measures to be taken during land preparation and construction at or above a certain slope percentage. The reasons for this include impact on land and structural stability, erosion, and surface water runoff. It is recommended to work with geotechnical and environmental engineers and local officials to ensure site and building development comply with regulations and do not negatively impact critical habitats

Development is subject to regulations at the local county and state levels, as well as the federal level and failure to take required mitigation measures can result in the rejection of necessary building permits. Permits and citations are generally issued at the local level with guidance at the federal level from the Department of the Interior (DOI) where necessary. 

 

Impacts of Stability of the Development 

Slope and aspect are determining factors in what use the land can serve. Renewable energy developers looking for a site to develop a solar farm of ground-mounted solar panels will seek out flat land; some local jurisdictions have laws restricting the development of solar arrays on slopes of a certain steepness. While rooftop single-home solar installations may sit at steeper slopes, ground-mounted solar arrays are generally on slopes of around 30 percent or less. Solar panels certainly work at steeper slopes; however, the solar installations themselves are limited in the amount of pressure they can support, necessitating alterations for steep slopes. Additionally, the erosion and drainage management required on steep slopes makes them cost and time-prohibitive for developers. Developers out of compliance with mitigation and inspection requirements for development projects on slopes of a certain gradient face citations and fines. 

The slope aspect of the land is also an important factor in terms of the solar radiation the land is exposed to. GIS analysts perform complete solar radiation analysis on land using 3-D mapping tools to register how surrounding topographical features and seasonal changes in the position of the sun impact solar radiation exposure on the land. In solar energy production, insulation is the amount of solar radiation on a surface during a certain period of time. Insulation properties on the land determine the profitability of solar installations; arrays that require too many alterations to the technology for the characteristics of the land will not be cost-feasible.

Impacts on Soil Nutrients 

The land’s slope and aspect characteristics and their impact on the solar radiation of the land also impact the content of the soil. Properties such as the dryness and moisture of soil are affected by solar radiation exposure as well as by wind speed. The steepness of the slope impacts the distribution of sediments; gravity forces sediment to the lowest point, resulting in thin soil layers at the highest areas of the land. Slope and aspect are determining factors in the presence and abundance of nitrogen in the soil. Nitrogen is an important element for plant growth and occurs in many compounds in soil. In the study Effects of slope morphology and position on soil nutrients after deforestation in the hilly loess region of China, by Zhang et al., the soil of northern facing slopes is a cooler temperature, more moist, has more abundant soil nutrients, and accordingly have more vegetation cover than slopes with a southern aspect. Also, due to the effects of gravity, different positions on the slope will experience runoff at different speeds. The higher speed flow at high points of the land results in greater erosion from high and middle sections of slopes, contributing to the accumulation of nutrients at the low points.

Particularly in relatively dry regions with arid to semi-arid climates, hillslops and aspect is a determining factor in the area’s microclimate. Microclimate is made up of air and soil temperature, windspeed, and evapotranspiration, the evaporation of water contents from soil and plants in the area. The microclimate, in turn, determines the amount of available elements such as nitrogen and the organic matter content of the soil. In the study The effect of slope aspect on vegetation attributes in a mountainous dry valley, Southwest China by Yang et al., it was found that woody plant species had larger structure attributes (biomass, height, and vegetation cover) on northern slopes than on southern-facing slopes, largely due to the amount of available nitrogen and phosphorus in the soil as a result of the microclimate and solar radiation. This means that more plants covered more land area and grew taller on northern-facing slopes, an important consideration for the development and maintenance of any site. 

Land use and past management also have an impact on soil properties, erosion, runoff, and water quality. In a study on hillsides in Ethiopia, it was found that land use in combination with the slope and aspect of the hillside were important factors in how erosion and runoff affected the soil content. Soil from slopes of a northern aspect had greater silt and sand content, while western aspect slopes had a greater presence of clay soil. It was also found that soil organic content was greatly impacted by slope; western and northern slope aspects were associated with the highest levels of soil organic content (SOC), which is consistent with the greater vegetation cover on these slopes and their favorability for agricultural development in comparison with southern and western facing slopes, which were found to have lower soil organic content.

Runoff Impacts on Watershed 

Understanding how the slope and aspect of land affect erosion and runoff patterns is important due to the impact of runoff on water quality in the local watershed. Watersheds are made up of a body of water such as a lake, river or stream and the surrounding land that drains into that water source. Water runoff is driven by gravity to this water source, which may also be an underground source such as an aquifer, where polluted runoff can compromise water supplies and the ecosystems that depend on them. Runoff is a natural process that replenishes water features, but industrial and particularly agricultural runoff can contain excess nutrients such as nitrogen and phosphorus. Excess nutrients in water supplies are linked to harmful algal blooms, which can even be toxic to the point of making people ill through contact recreation and are known to have killed dogs and wildlife that swim in and drink affected waters. 

Forested watersheds absorb and filter water by absorbing large amounts of water into the soil relatively quickly. Unforested landscapes experience greater amounts of soil erosion as a result of runoff, stripping the soil of nutrients and depositing sediment downstream. Another type of natural infiltration, the process of water absorption, includes infiltration from play lakes. Developed watersheds are also likely to experience a negative impact on watershed water quality due to contaminants picked up on neighborhood streets and urban drainage systems.

Methodology: Slope and Aspect Survey

GIS analysts utilize spatial analysis software ArcGIS to digitally map slope and aspect along with other information about the land, such as contours. Using this software and digital elevation models, trained GIS professionals can calculate slope and aspect without physically setting foot on the land by using an algorithm that works with elevation data. Within ArcGIS, Catena is a data management and web-accessible spatial analysis tool. Catena is also known as the Environmental Resource Assessment and Management System (eRAMS) and provides information from remote sensing technologies, allowing the study of vast spaces from anywhere. 

This geospatial technology can be applied to study pollution using the land-use regression model (LRU), which is widely applied to studying air pollution. Regression models in ArcGIS can be used to evaluate differences in pollution concentrations and better understand how spatial factors such as topography and development influence those concentrations. This software is also useful for modeling the effect of mitigation solutions and modeling the sustainability of development plans.

Who Performs Slope and Aspect Calculations? 

 

Land Surveyors

 Land surveyors are professionals that may either physically measure land or may use satellite imagery to produce an exact map of a parcel of land. Surveyors on the ground use theodolites, specialized surveying equipment that measures the horizontal and vertical angles between points. Surveyors often demarcate property boundaries and document the initial lay of the land before most development projects begin.

 GIS Software 

GIS software such as ArcGIS uses information gathered through remote sensing to generate a 3-D model of the land, which can be manipulated and tethered to data input by GIS analysts to understand the impact of slope and aspect on land use. While GIS Software gathers all the relevant information, an analyst is still needed to make and analyze calculations. 

Transect 

The Transect Marketplace offers the ability to conduct the calculations needed for your project and add them to your Transect Report without breaking the bank.

The resulting slope and aspect layer will be added to the map and catalog in your Transect Report. A .kml of the slope is also created, given coloration, and made available for you to download to view in outside applications like Google Earth. 

When building in or near a city or urban county, local environmental regulations may trigger development constraints like wetland setbacks or additional surveys. Transect specialists will research county and municipal codes and regulations for rules and permits specific to natural resource constraints like wetlands, protected species, vegetation, and historic resources.

We will include the results of our research findings in the Regulations and Permits section of your Transect Report. The resulting regulations and permits are automatically included in future Transect Reports in the same geography.

To learn more about Transect, book a demo today!

Free Permitting Checklist

Practical Tips to Avoid Environmental Risk on all Your Projects

Download our environmental permitting checklist to get a step-by-step list of ways to protect your project from the 9 most common environmental risks.

Download Your Checklist