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: e_h1 = s_m/100
where s_{m =} map scale denominator (feet) and e_h1 = maximum allowable ASPRS Class 1 root mean square error (RMSE).

Example: e_h1 = 200 map scale / 100
e_h1 = 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: e_s1 = c_i/6
c_i = contour interval and e_c1 = 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!