Tuesday, November 1, 2011

Defining a Topographic Survey using Photogrammetry

As a Professional Land Surveyor I have often used aerial photogrammetry firms to provide topographic maps for large scale projects or intricate urban sites that would be too costly to survey or map from the ground. Photogrammetry is a indispensable resource for any surveyor or engineer as they, and many other professionals, rely on aerial mapping to provide the base map for a variety of projects. The end product is generally a 3D CAD file representing the features and linework on the ground, a digital elevation model (DEM) representing the natural and manmade breaklines that are collected for accurate vertical surface data, and the orthophoto that is a geometrically corrected aerial photograph.

What is to be mapped, the specifications, and the scope of the project is determined by the surveyor or engineer when ordering the photogrammetry, so it is very important that he or she has a good understanding of the basics of photogrammetry and what to expect from the topographic map.

To satisfy my own interest and to help others understand the basics of specifying requirements for aerial mapping I have put together the following information for reference.

Selecting a Map Scale (plotting scale) and Contour Interval – The required mapping scale and contour interval will determine the detail of the feature collection and the accuracies for the collected data. The tables below can help in selecting a map scale based on project requirements:

Map Scale Planimetric Accuracy for Well-Defined Points RMSE (feet)
ASPRS Class 1 NMAS ASPRS Class 2
1"= 20' 0.2 0.3 0.4
1"=40' 0.4 0.6 0.8
1"=50' 0.5 0.75 1
1"=100' 1 1.5 2
1"=200' 2 3 4
1"=400' 4 6 8
1"=800' 8 12 16

American Society for Photogrammetry and Remote Sensing (ASPRS)
Horizontal Inaccuracies
Comparable Horizontal inaccuracy allowances stated by the ASPRS standards for Class 1 mapping can be determined by using the following equation: eh1 = sm/100
where sm = map scale denominator (feet) and eh1 = maximum allowable ASPRS Class 1 root mean square error (RMSE).
Example: eh1 = 200 map scale / 100
eh1 = 2 feet
Contour Inaccuracies
ASPRS Class 1: ec1 = cr/3
ci = contour interval and ec1 = maximum allowable ASPRS Class 1 error.

Spot Elevation Inaccuracies

ASPRS Class 1: es1 = ci/6
ci = contour interval and ec1 = maximum allowable ASPRS Class 1 error.

What features will be collected? The examples below reflect typical features collected at various Map Scales. As the map scale increases the feature detail typically decreases.

Collected Features at 1"= 20' Map Scale (example)
         Bridges           Gates          All stairways
         All buildings           Swimming pools          Curb & Gutters
         Railroad centerlines          Poles          Paved walkways
         Drainage edges           Power poles          Sidewalks
         Drainage channels           Flagpoles          Paved driveways
         Waterlines          Signs           Grass lines 
         Dams          Call boxes          Handrails
         Vegetated areas          Cell sites          Street furniture 
         Roads                                Towers          Parking bumpers
         Jeep trails          Windmills          Speed bumps
         All decks          Drains          Posts
         Pipeways          Headwalls          Planters
         Individual trees          Culverts          Wells
         Concrete slabs          Fountains          Sprinklers
         Footpaths          Cattle guards          Mailboxes
         Foundations          Manholes          Small utilities 
         U/C buildings          Lights          Valves
         Trailers (non-mobile)          Electrical Boxes          Standpipes
         Covered areas          Rail signals          Pavement paint
         Ramps          Fire hydrants          Pole anchors
         Fences          Golf courses          Swamp areas
         Walls          Golf cart paths          Rock areas
         Dirt piles          Orchards

Collected Features at 1"= 40' & 1"=50' Map Scale (example)
Road edges Gates Rock areas
Bridges  Swimming pools Dirt piles 
All buildings  Poles Large stairways
Railroad centerlines Power poles Gutters
Drainage edges  Flagpoles Paved walkways
Drainage channels Signs and billboards Sidewalks 
Waterlines Call boxes Paved driveways 
Dams Cell sites Grass lines 
Vegetated areas Towers Handrails
Jeep trails Windmills Street furniture 
Large decks Drains Parking bumpers
Pipeways Headwalls Speed bumps
Individual trees Culverts Posts
Concrete slabs Fountains Planters
Footpaths Cattle guards Wells
Foundations Manholes Sprinklers
U/C buildings Lights Mailboxes
Trailers (non-mobile) Electrical equipment Small utilities
Covered areas Rail signals Valves
Ramps Fire hydrants Standpipes
Fences Golf courses Pavement paint
Walls Golf cart paths Swamp areas

Collected Features at 1"= 100' Map Scale (example)
Road edges U/C buildings Drains
Bridges  Trailers (non-mobile) Headwalls
Buildings (no sheds) Covered areas Culverts
Railroad centerlines Ramps Fountains
Drainage edges  Fences Cattle guards
Drainage channels  Walls Manholes
Waterlines Gates Lights
Dams Swimming pools Electrical equipment
Vegetated areas Poles Rail signals
Jeep trails Power poles Fire hydrants
Large decks  Flagpoles Golf courses
Pipeways Signs and billboards Golf cart paths
Individual trees Call boxes Orchards
Sidewalks, driveways Cell sites Swamp areas
Footpaths Towers Rock areas
Foundations Windmills Dirt piles

Before requesting mapping one should consider the company or persons with whom they will be contracting with to complete the mapping project. Ask if the firm has certified photogrammetrist and certified technologist, such as an ASPRS certification, this tells me that they take their profession seriously - especially if they require certification as a company wide standard.
Do some due diligence and ask questions. Do they use digital softcopy methods? Are they up-to-date with their aerotriangulation and compilation methods? Do others speak highly of there product and service? What is important to you – is it Quality? Level of Understanding? Timeliness? – hopefully it is more than just cost of the mapping alone! You’re the one certifying the design or survey based on this mapping!

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