Week 10: Land surface modelling Flashcards
3 representations of earths shape
- Terrestrial surface
- Geoid
- Ellipsoid
What is terrestrial surface in terms of earths shape
Earths actual shape
What is geoid in terms of earths shape
Sea level - a complex mathematical estimation of the earths shape
What is ellipsoid in terms of earths shape
Simple mathematical estimation of the earths shape
For engineering works we generally consider that we are working on ______ unless we are working on a ______
A planar surface,
Very large area
Why is there a change in any planar surface across the globe
Because of the curvature of the earth, there is no perfectly flat plane on earth
Radius of the earth
6,371 km
When designing/building over a large area, we would need to determine the
Topography: What the existing shape of the land surface is
What are we determining when we look at topographical info
- The ground surface details in relation to some repeatable measurement system
- Northings (y value)
- Eastings (x value)
- Heights (z value)
The spatial positions of features are
The differences in the X, Y and Z positions between the features
Surveyors also look at how the topography changes over time. This could be due to
- Natural process of weathering and erosion
- Design where we modify the land surface for a specific purpose
Reasons that an existing surface would need to be modified
- Create level playing fields
- Provide platforms for construction purposes
- Create grades so that stormwater can be directed and collected within a network of pipes
- Create access to properties and destinations through a roading network
- Removal of trees - Construct rain gardens or bioswales
How do we collect the information that will allow us to understand the topographical information of the land surface
- By field surveying methods
- By using GPS/GNSS
- By using different lazar scanning technologies
- Lidar scanning
What does Lidar stand for
Light detection and ranging
Field surveying methods process
- Usage of total stations and prisms
- Picks up individual points and measures the change in position and height from the known position
- May also be completed by a robotic total station
GPS/GNSS methods process
- Picks up individual points and measures the change in position with a base station set up over it
- Or can measure individual points in relation to a wider coordinate system
Lazar scanning methods process
- Picks up the reality of the scene that is being looked at by scanning millions of positions relative to each other, creating a dense point cloud
- Because the scan picks up so much information, the point cloud needs to be interrogated to get the required information and be geo-referenced into a known coordinate system
Once the positional data is collected, how do we change it from coordinates (just points) into a useful and usable form
- We change the data into a representation of the shape of the land surface
- This representation is usually a topographical plan that includes all of the height information and features across the site
If we were surveying the North Ground, what features would we want to pick up
Around the outside: The streets, The kerbs, The widths of the footpaths
Inside the ground: Trees, Clubrooms, Features (lamp-posts, benchs, etc)
And the heights associated with each
On a topographic plan, we are looking down onto a surface and this is called
The plan view, where all distances are measured in horizontal
How can we represent height information on a topographical plan so that the actual surface information is available in all three dimensions
- Add height information to every measured point
- Use contours (convention heights to a metre or common multiple)
What is a contour
A line on a map or plan that joins points of equal height above a known datum
Contours depict
The relief of the terrain, that is the changes in height that occur across the area of interest
Contour intervals are
The vertical distance between level surfaces forming the contours (varies depending on the maps purpose scale and diversity of relief)
What is the best way of collecting the position and height information on North Ground
Around the outside where all the trees are: A direct method which is suitable for any terrain
On the open field part:
A grid method where points are measured conform to a square or rectangular grid
What would be the danger if we only used a single method of collecting the position and height information
We would miss important information and would not be able to display a true representation of the shape of the surface (where field diagrams are really useful)
When we have collected all of the data, and using appropriate software, we are able to create
A digital terrain model (DTM), also sometimes known as a digital elevation model (DEM)
A DEM is a
3D representation of continuous variations of relief over an area
Some assumptions need to be made in order for DTM/DEM information to be generated, these include
- Usage of triangles created between points which develops into a derived triangulated irregular network (TIN)
- We assume that all triangle sides have a constant slope
- We assume that the surface area of any triangle is a plane
The triangles in DTM/DEM are created to
Join up the points
The contours are created by
Interpolation along the triangles sides
What additional information is required when creating topographic plans and contours
Features such as rivers, streams, and roads, as they are linear topographic features that include uniform slops and extended locations
Features such as rivers streams and roads need to use
Breaklines (discontinuity lines) to show top of banks, bottom of banks, walls, other roads, etc
Breaklines force
Triangle sides to from along the feature, creating correct representation of the ground
Two methods to improve the look or shape of the surface (terrain)
- Edit the DEM (underlying information)
- Shape or smooth the contour lines
Editing of the DEM to improve the look of the surface is done by
- Swap triangles to change shape of terrain
- Delete triangles (skinny ones round edges)
- Edit heights of points (small changes)
Now that a topographic plan with contours exists, it can be used for analysis of the _____ by looking at _____
Shape of the surface (terrain), Different views
In civil engineering and development work, the views that we are interested are
- Plan view
- Longitudinal section
- Cross section
Longitudinal section description
A profile taken along the proposed centreline at regular intervals (20m) showing existing ground levels and the possible constructed levels (formation level)
Longitudinal sections are used to determine
- If suitable grades are available along the length of the long section
- The difference between the existing ground and the information level
The difference is known as cut or fill and is dependant on the differences in levels between the surfaces
How are suitable grades on a long section calculated
By the vertical distance (rise, positive or negative) divided by the horizontal distance (run)
Longitudinal sections allow you to find out
How much work will be required along the centreline (or other selected position) and determine if the amount of cut or fill is acceptable
Longitudinal sections do not show
Any RLs outside of the centreline, information is only available for the slice being shown
Longitudinal sections are determined and drawn by
Establishing the profile that you want to see and then drawing it from the side elevation perspective
In longitudinal sections, the profile position is established on
The plan view
Longitudinal sections follows the line that you won’t be able to see. This means it can be
A straight line, a curved line or a mixture of both
The long section establishes the horizontal distance along the x axis of a plan and the
Corresponding heights on the Y axis
In long sections, the existing heights are determined from
The contour information on the plan view
In long sections, the data panel establishes
- A datum for the drawing
- Provides line spaces so that information about the following is provided at the regular interval positions
A datum for the drawing is
A position below the lowest level that will be shown on the long section
A datum for the drawing allows for
The best use of the available space and removes large gaps of no useful info on the plan
Formation level
The desired height levels along a long section
Cut or fill is the difference between
The existing ground level and the formation level
Having different scales is called a
Distorted scale
Usually a what scale difference between vertical and horizontal scale and why
1:10 scale difference because change in the vertical is generally much less than the distance represented in the horizontal
If we used the same scale for both horizontal and vertical, we would not be able to
See the changes in elevations and this would hinder our design abilities
Anything that appears on the profile line should be shown on the
Long section
Features that should be shown on the long section if they are on the profile line include
- Underground services (pipework)
- Features on the surface
- Features above the surface (may interfere with design work)
Cross sections are
Profiles that are taken at the same regular intervals as used for the longsection but perpendicular (at right angles) to the centre line
Cross sections are used to establish
- The detail at each of the interval position
- The existing ground, in the cross section profile, at each position (comes from the long section)
Cross sections are drawn from
The centre line position
In cross sections, the extent of the construction surface extends out until
It meets the existing ground (catch point)
Cross sections are needed to
- Determine the extent of the cuts or fills
- Calculate the volume of the earthworks
- Volume of earthworks is the total amount of work that needs to be completed
- Requires a typical cross section to be created
Since we use the plan view and the long section to draw a cross-section, the same assumptions regarding ______ must remain
Linear relationships
Cross sections are drawn at
A suitable natural scale
A natural scale means
- The horizontal and vertical scales are the same
- No distortion of the changes in height
Why are cross sections drawn at a suitable natural scale
In the long section, it is likely that you will never be able to see the entire length of the project so you will never be able to see the full job in front of you
What data was collected from the three created views
Point position information, x y and z infoTh
Having three different drawings is important because
- They give three different views on seperate drawings and are different representations of the same data
- All three views must be used together in order to understand the shape of the surface and allow complete analysis of a project to occur
How can the amount of work to be computed be determined from the info contained in the three views
We calculate the volume of earth works that will be required for the changes between the surfaces (existing ground and the new surface (formation level))
3 things that the plan view provides us with
- The position of all the topographical features and the spatial relationship between them
- Height information through the depiction of contours
- Only horizontal distances
2 things the longitudinal section provide us with
- The info of the entire length of the provided shown from a side profile
- Horizontal distances and vertical distances, often at a distorted scale to add emphasis to the changes in height
2 things the cross-section provide us with
- The info of a position from a perpendicular position to the centreline
- At a natural scale
3 things the combination of the three views allow to happen
- The plan view allows us to establish the shape of the existing surface
- The info on the plan view and the long section allows us to create the cross-sections
- As soon as we have the cross sections we use the info to calculate volumes
The important information is being able to determine an area from the cross-section, and once you have a series of cross-sectional areas you can
Calculate the volume
Volume between the adjacent cross sections is the
Average area of sections x the distance apart (method of average end areas)
What does the method of average end areas calculation rely on to be accurate
The assumption of the linear relationship between the sections
All calculations will be slightly inaccurate, we therefore need to
Find out how inaccurate we are and let anyone who is associated with the calculations know that there is an error associated with the value
What are some uses for our ability to define and show the shape of surface
- Investigation
- To check the suitability of the site
- Design
- Calculations
- Construction
- Perspective
- Supervision/monitoring
- Effects
