Modelling and reading topographic map becomes more convenient with the aid of modern technology such as online map surveying and 3-D modelling software.
Most contour lines found in online topographic maps such as Google map (https://www.google.com.ph/maps), Open Street Maps (https://www.openstreetmap.org/), NAMRIA (http://www.namria.gov.ph/), etc. are labelled with elevations. These topographic elevations found mostly on online map can only be viewed in a macro scale level, showing a minimum of 20m contour intervals when zoomed in.
Topographic maps may have many contour lines. Technically, we can increase the number of contour line and adjust the elevation or contour interval using different 3-D modelling software such as SketchUp.
In this tutorial, we will learn how to adjust the contour intervals using SketchUp. Here, we will try to generate 5 meter contour interval based on the Google map topography, with the specified value of 20 meter contour interval.
Before we start the modelling procedures, we should analyze first the contour interval of the topographic map. Here’s a quick guide on how to read topographic map.
QUICK GUIDE ON HOW TO READ TOPOGRAPHIC MAP
|(1) Index Contour, (2) Minor Contour Line, (3) Contour Interval|
To determine the contour interval of a point, you must have a reference line from which to start. This reference line is an index contour (major contour line). Based on the reference map, an index is one that is bolder, darker contour line and has a value written on the line. The value written on the index line is the elevation.
Every topographic map has a contour interval which remains constant throughout the drawing. The elevation value of the minor contour lines can be determined through the counting process from one index contour to another. For instance, if the first index contour line has a value of 100m and the second index contour has a value of 200m, the value of the minor contour lines which can be found between the two index contour lines will be 120m, 140m, 160m, and 180m. Therefore, the contour interval is 20 meter.
|20 Meter Contour Interval|
6 CARDINAL RULES OF CONTOURS
RULE #1: Contour always occur in pairs
RULE #2: Contours never cross
RULE #3: Contours have equal vertical separation
RULE #4: All contour lines close in themselves
RULE #5: Contours do not merge
RULE #6: The steepest slope is a line perpendicular to the contour
A. HOW TO VIEW GOOGLE MAP WITH CONTOUR LINES
STEP 1: Open the Google map> Click the expand side panel>search the location of the site by typing the address on the search bar> click the 'map view'. In the figure, you see the results for Angono-Binangonan Petroglyphs.
STEP 2: Click the menu from the Google map panel> click the terrain option
B. HOW TO CREATE A 3-D TOPOGRAPHIC MODEL USING SKETCH-UP
STEP 1: Click 'File'> import image> select the map image> click 'Open'
STEP 3: Draw a 1000 meter x 1000 meter border line box
STEP 4: Click Arcs from the drawing tool> Draw and plot the contour lines> delete any faces that formed when lines were drawn
STEP 6: Ctrl+A (Select all). Using the Sandbox Tool, click the icon ‘From Contours’ or go to Draw>Sandbox>From Contours. This process will automatically generate 3D object(s).
RESULT : 3-D Topographic model with 20 meter contour interval
C. HOW TO CREATE A 5 METER CONTOUR MAP USING SKETCH- UP
Here, we will try to adjust the contour interval of our 3-D model terrain from 20 meter (Google map contour reference) to 5 meter contour interval.
|STEP 1: Lock the grouped terrain model and the grouped index contour lines. To do that, select the model> right click> click 'Lock'|
STEP 2: Draw a square or rectangular plane> make group> move the plane to the lowest point of the terrain
|STEP 3b: Type 50x (to duplicate the plane)> press Enter|
STEP 4: Select the entire plane> Right click > make group> Hide
|STEP 5: Unlock the terrain model. To do that, select the model> right click> click Unlock.|
|STEP 6: Explode the terrain model. To do that, select the model> right click> click Explode|
|STEP 7: Unhide the grouped plane. To do that, go to Edit> select 'unhide'> click ‘Last’|
|STEP 8: Select the grouped slicing planes> Right click> choose ‘Intersect Faces’> click ‘With Model.’ This process will enable you to intersect your grouped slicing planes with your terrain model.|
STEP 9: Delete or Hide the grouped slicing planes.
Result so far
STEP 10: Based on the “6 Cardinal Rules of Contours”, all contours never cross or merge with each other. Thus, we need to remove all the overlapping edges from the model. To do that, hold Ctrl+ Erase Tool and remove all the overlapping edges.