When performing an airspace analysis, it can be come necessary to look beyond the results of the analysis summary reports to determine if the proposed construction location is in fact “acceptable” or “not acceptable.”

Specifically, I am referring to the “hidden impact” Existing Natural Terrain may have upon the results of the Airspace Analysis.  Therefore, we have introduced a new Slope Line tool to assist the Airspace user in determining if and where the natural terrain impacts or “intersects” an aeronautical sloping surface such as a 40:1 Departure surface.

NOTE:  The steps listed below describe the use of the Slope Line tool; however, several assumptions have been made.  The instructions below assume that a topographic image has been loaded and calibrated as described in the Airspace Survey manual, and that appropriate approach or departure trapezoid has been overlaid upon the topographic image and the applicable study point has been plotted.   Should you need assistance with these steps, please refer to the Airspace Survey manual contained on your Airspace Software CD in the “Manuals” folder for detailed instructions.

How to Set-Up the Slope Line Tool:

7. Next, the user may manually enter the “Start Coordinates” elevation using the the computer keyboard if they have access to this elevation data. If the user does not have the elevation data, it can be electronically retrieved from a USGS server with a single click the left mouse button on the caption “Elevation” in the Start Coordinates section of the Slope window. This will allow the survey program to automatically retrieve the elevation of the point 
selected in steps 5 & 6 above.

8. An elevation window will open displaying the USGS elevation data for the Start Coordinates.  Single click the left mouse button on the [OK] button, and the Elevation field will automatically be populated with the USGS elevation data.

How to Read the Slope Line Tool:

9. Drag the cursor along the surface of the topographic map stopping on a contour line within the departure trapezoid.  For this example, the cursor was stopped on the contour line 1000.  Looking in the Range section of the slope window, the Elevation indicated is the maximum allowable height AMSL at the location where the cursor was placed.

Note: This “elevation” is automatically calculated based upon slope selected and distance from start coordinates. Accordingly, this “elevation value” changes as the cursor is moved across the surface of the topographic map.

For this Example… at the location marked with the Black + the max height is 1006.47 feet AMSL.

How to Apply the Slope Line Tool:

10. Repeat steps 5-9 until the areas where the ground meets or exceeds the aeronautical surface being analyzed are identified and marked.

In this example what the Airspace analyst is looking for, by using this tool, are places within the departure trapezoid where the natural terrain exceeds the maximum allowable height indicated in the “Elevation” window of the Range section of the Slope Line tool.

In other words, if the “elevation” displayed in the Range window is less than the elevation displayed on the contour line for that point on the topographic map, then the natural terrain penetrates the aeronautical surface.

Each time an area of impact is identified, the analyst may click the left mouse button and then select the [Mark] button on the right side of the slope window.  This will place an “X” at the location. 

Once all the areas of impact are identified, the analyst can use the Draw tools in Survey to “map out” the area of Existing Natural Terrain Impact and provide this information to their engineering department or other internal or external customers (see image below).