Which type of surveying would be best for my project ... › wp-content › uploads › 2019 › 07 › ...Satellite surveying requires no in-country presence or permits LiDAR surveys

Which type of surveying would be best for my project: PhotoSat, LiDAR, Drone, or GPS?

by Gerry Mitchell, P. Geo, PhotoSat President Vancouver,2018

580 – 1188 West Georgia St. Vancouver, BC, V6E 4A2, Canada | +1.604.681.9770 | www.photosat.ca

Jump to:• Situations with Clear Choices• For other situations consider the following factors• Elevation surveying accuracy comparison • Survey point density comparison • Survey reliability comparison • Surveying bare ground elevations in areas of vegetation

cover• Timing comparison• Cost comparison• Case history comparisons• PhotoSat experience with LiDAR, drone and GPS survey

data

For volume measure-ments of small areas near urban centers, use drone surveying

For large areas in remote locations, use PhotoSat surveying

For bare ground sur-veying in areas with regions of continuous tree cover, use LiDAR surveying

For surveying of up to about 50 ground fea-tures, use differential ground GPS

For surveying of up to about 500 ground fea-tures, use Real Time Kinematic (RTK) GPS

We are often asked about the best survey methods to use for projects. As with most technology issues, the answer depends on the situation.

Situations with Clear Choices:

• areas up to 4 km2• ittle or no vegetation cover • near professional surveyors or engineers who con-

duct drone surveys

• Areas of tens or hundreds of square kilometers• Remote areas such as the high Andes of South

America, the oil fields of Iraq or the arctic islands• Sparse or no vegetation

• Areas of tens or hundreds of square kilometers• When you need better than 50 cm bare ground ele-

vation survey accuracy beneath tree cover• Areas must be accessible for small survey aircraft

• Up to about 50 ground points or features• Points accessible by road and up to 500 m easy

walking• Differential GPS surveyors can usually survey about

12 points per day

• Up to about 500 ground points or features within approximately 10 km of RTK base

• RTK rover must be in radio contact with the GPS base

• Establishing the RTK base coordinates may take one day

• RTK GPS surveyor can survey each point in less than one minute

• Production depends on walking or driving time to access the points


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GPS surveying accu-racy: 2 cm to 10 cm in elevation

LiDAR surveying accu-racy: 5 cm to 15 cm in elevation

PhotoSat surveying accuracy: 15 cm to 30 cm in elevation

For other situations consider the fol-lowing factors:

• Elevation survey accuracy• Survey point density• Reliability • Presence and type of vegetation • Contractor’s track record• Timing• Cost

Elevation surveying accuracy compari-son

• Differential GPS surveying accuracy: 10 cm in eleva-tion

• Real Time Kinematic GPS surveying accuracy: 2 cm in elevation

• High quality LiDAR surveying accuracy: 5 cm in ele-vation

• Usual quality LiDAR surveying accuracy: 15 cm in elevation

• One ground control survey point per 100 km2: 30 cm in elevation accuracy

• Ten ground control survey points per 100 km2: 20 cm in elevation accuracy

• Over 100 ground control survey points per 100 km2: 15 cm in elevation accuracy

• PhotoSat uses a proprietary geophysical stereo sat-ellite processing method and system derived from oil and gas seismic processing. To our knowledge, no other satellite processing methods match the Photo-Sat satellite survey accuracy. We have published nu-merous comprehensive satellite surveying accuracy studies on our website and at technical conferences. Some of these accuracy studies are available on the PhotoSat webSite.


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http://PhotoSat.ca

Drone surveying ac-curacy: 5 cm to 1 m in elevation

PhotoSat surveys are the same everywhere in the world

LiDAR surveys are generally good qual-ity; contractors must be carefully consid-ered

• High quality drone surveying: 5 cm in elevation accuracy

• Good quality drone surveying: 20 cm in eleva-tion accuracy

• Usual quality drone surveying: 1 m in elevation accuracy

Survey point density comparison

• LiDAR, PhotoSat and drone survey points: 1 m by 1 m, or 50 cm by 50 cm centers

• RTK GPS surveys for volume measurement points: usually 10 m by 10 m centers

Survey reliability comparison

PhotoSat surveys use the same satellites, processing technology and team for surveying projects everywhere in the world. This provides consistent high-quality re-sults for all projects.

LiDAR surveys are carried out by a variety of contrac-tors with different LiDAR surveying equipment. The majority of LiDAR survey contractors in Canada, the US, Western Europe and Australia provide good quality LiDAR survey data. You need to learn the qualifications and track record of each LiDAR surveyor.

LiDAR survey contractors in the developing world have a wide variety of expertise and experience. We have used very good quality LiDAR survey data from the developing world, but we have also seen LiDAR survey data with elevation errors of more than 20 m.


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Drone surveys have a wide range of reliabil-ity; contractors must be carefully consid-ered

Differential GPS sur-veys; important to re-view digital recordings

Real Time Kinemat-ic (RTK) GPS points must be in direct radio contact with the GPS base

Drone surveys have a wide variety of elevation survey-ing accuracy. You need to check the qualifications and track record of each drone contractor. We have used drone survey data accurate to 10 cm in elevation, but we have also seen drone survey data with elevation errors of over 5 m.

It is essential that the drone contractor includes a quali-fied surveyor or professional engineer on staff. They will understand survey benchmarks, projections and hori-zontal and vertical datums. There must be an accurate survey benchmark and accurate ground control survey points on each drone survey project. Drone surveyors who use drones with Real Time Kinematic (RTK) GPS systems require fewer ground control survey points to achieve good survey accuracy.

GPS signals must be recorded for sufficient times at the GPS base and survey point to achieve the required ac-curacy. For 10cm elevation survey accuracy, each sur-vey point should be recorded for 30 minutes plus one minute for each kilometer of distance between the GPS base and the survey point. You should request copies of the GPS recordings in RINEX format from the survey contractor. These will provide an unambiguous check of recording times and accuracies.

It is essential that the GPS contractor includes a qual-ified surveyor or professional engineer on staff. They will understand survey benchmarks, projections and horizontal and vertical datums and other essential sur-vey procedures and conventions.

The points to be surveyed with RTK GPS must be in locations that are in radio contact with the RTK base station. It may take a day of GPS recording to establish accurate global coordinates of the GPS base station. So long as the GPS Rover is in direct radio contact with the GPS base, points can be surveyed to within 2cm accu-racy in a few seconds.


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LiDAR is the best tech-nology for surveying bare ground beneath vegetation cover

PhotoSat bare ground elevations beneath tree canopy are esti-mates

Drone survey bare ground elevations be-neath tree canopy are estimates

Surveying bare ground elevations in areas of vegetation cover

LiDAR surveys are capable of surveying bare ground elevations beneath continuous forest cover. Fewer photons reach the bare ground in the vegetated areas than in the areas of exposed bare ground. Except in cases of extremely dense tropical jungle canopy, a few laser photons penetrate the forest cover and reach the ground. Consequently, LiDAR survey points beneath vegetation cover are subject to more error than LiDAR points in areas of exposed bare ground.

To produce bare ground elevation surveys in forested areas, PhotoSat first produces and adjusts a model of the tree canopy height. This tree canopy model is then subtracted from the satellite survey of the top of the tree canopy.

Wherever there are openings in the tree canopy, Pho-toSat measures the tree heights. When there are tree canopy openings about every 100 m, the PhotoSat estimates of bare ground elevations beneath the tree canopy are usually accurate to about 10% of the tree height. In these conditions, with trees up to 20m

In areas of continuous tree canopy with very few open-ings to bare ground the PhotoSat estimates of bare ground elevations are usually accurate to better than 10 m in elevation. The accuracy of drone survey bare ground elevations beneath tree canopy will depend on the drone survey-or’s method of estimating tree canopy height. Different drone surveyors use different processes and software systems to estimate the height of the tree canopy.


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GPS surveying be-neath continuous tree canopy is difficult to impossible

PhotoSat delivers 100 km2+ mine site sur-veys within five days of the satellite pass, most projects deliver within two weeks

Satellite surveying requires no in-country presence or permits

LiDAR surveys re-quire aircraft access that can take days or months

LiDAR contractors typically take months to complete and deliv-er the processed bare ground elevation data.

The tree canopy blocks and distorts GPS signals so GPS surveying through forest cover is usually difficult to impossible.

Timing comparison

For repeat surveying of mine sites, PhotoSat delivers critical mine areas such as sand dumps within two days of the satellite pass. The entire mine site survey is usu-ally delivered within five days of the satellite pass.

For 100 km2 areas that are being surveyed for the first time, PhotoSat usually delivers the survey data within two weeks of the satellite pass.

The timing of the satellite photo acquisition depends on cloud cover and the competition for access to the sat-ellites. Arid areas of Mexico usually collect within two weeks. Perpetually cloudy areas such as Columbia can take up to a year to collect.

There is no requirement anywhere in the world for per-mits to acquire satellite photos. Consequently, PhotoSat can survey everywhere in the world with no local per-mits or presence.

For survey areas in the same country as the LiDAR aircraft, access time can be a few days. For survey areas in some parts of the developing world, the LiDAR aircraft must cross international borders. This requires local permits to survey. In some countries, it can take months to acquire the necessary aircraft survey per-mits.

You may be able to specify shorter data processing and delivery times in your LiDAR survey contract.


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Drone surveys of up to 4 km2 can be pro-cessed and delivered the next day

Drone surveys of over 100 km2 can take months to acquire and process

GPS survey data can be available the day after acquisition

PhotoSat survey of 100 km2 costs about $20,500 USD every-where in the world

For drone surveys of areas of up to 4 km2, the acquisi-tion and processing of the drone data can be completed in one or two days. The establishment of an indepen-dent GPS survey benchmark accurate to 10 cm may take an additional day.

A case history of a 150 km2 drone survey in Madagas-car, published by the senseFly drone company, appar-ently took 90 days to acquire and 120 days to process. This case history was available on the senseFly website in September 2016. This case history is discussed fur-ther below.

When there is an established benchmark to use as a differential GPS survey base, the GPS survey data can be processed and delivered the day after the field sur-vey. Field survey timing will depend on GPS crew avail-ability and the time to reach the project area.

When a new benchmark must be established by re-cording 12 hours of GPS signals, global accuracy of 20cm for the benchmark can be achieved the next day. 2 cm accuracy for the benchmark requires the precise ephemeris GPS correction data. This is available two weeks after the date of the GPS acquisition.

Cost comparison

In September 2016, a PhotoSat survey cost $13,000 USD for 25 km2, $21,500 USD for 100 km2 and $140,000 USD for 1,000 km2. The accuracy is between 15 cm and 30 cm in elevation depending on the densi-ty of ground control survey points. These costs include everything except the ground control point surveying. The deliverables are a 1 m by 1 m survey grid, a 50cm ortho photo and an accuracy report.


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LiDAR survey costs vary widely due to the cost of aircraft mobili-zation

Drone survey costs for a 4 km2 survey within a one-day drive of the drone surveyor’s base seem to vary from approximately $1,000 USD to $6,000 USD

GPS survey costs vary widely across the world

RTK GPS surveys for volume measurements for contract mining in Western Canada usu-ally cost about $8,000 USD per km2

Since the cost of LiDAR survey aircraft mobilization and standby are so variable, no useful global generalizations can be made about LiDAR costs. LiDAR survey costs vary widely from coun try to country and from region to region.

In cost comparisons between PhotoSat surveys and LiDAR surveys, we have seen LiDAR survey costs vary from 50% higher than PhotoSat survey costs, to over ten times PhotoSat costs.

Drone surveyors range from single owner/operator businesses to global engineering companies. Costs vary with the quality of the drone hardware, processing software, and the qualifications of the drone operators and processors. Most of the drone surveys that we are familiar with are for areas less than 4 km2.

GPS survey costs are usually based on a day rate for a two or three-person GPS survey crew with a vehicle and GPS survey equipment. This cost varies from a few hundred to a few thousand dollars per day depending on the country and region.

For contract mining in Western Canada, companies use measured volumes as the basis for the contract pay-ments. The convention in many of these contracts is to measure the mined volumes with 10 m by 10m RTK GPS survey grids before and after the mining. This re-quires 10,000 GPS survey grid points per km2. The cost generally turns out to be about $4,000 USD per km2 for each of the survey grids before and after the min-ing, for a total cost of $8,000 USD per km2.


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Madagascar Drone survey, 150 km2, 90 days acquisition 120 days processing, cost not reported:

Comparable 2015, 357km2 PhotoSat survey in Peru, 8 days acquisition, 11 days processing, cost $55,000 USD

Case history comparisons

The Swiss drone manufacturer, senseFly, had a case history on its website in September 2016 of a 150 km2 drone survey for a graphite project in Madagascar. This survey was carried out by Energizer Resources of Toronto Canada using two senseFly drones. Area: 150 km2 Ortho photo pixel resolution: 10 cm Elevation mapping accuracy: 20 cm Number of ground control survey points: 70 Field acquisition: 90 days with three people for a total of 270 people days Data processing: 120 days Total cost: the only information in the report on cost is as follows: “The price (of the drones) was very low compared to a LiDAR survey, which would have cost hundreds of thousands of dollars”

Area: 357 km2 Ortho photo pixel resolution: 50 cm Elevation surveying accuracy: 20 cm Number of ground control survey points: six Satellite acquisition: eight days after satellite tasking (the actual stereo satellite photo acquisition time is about one minute) Data processing: 26 days elapsed time, processing time 11 days Total cost: $55,000 USD (does not include the cost of surveying the six ground control points which probably cost less than $5,000 USD)


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LiDAR surveys are usually accurate

Over half the drone surveys have had sig-nificant survey errors

PhotoSat experience with LiDAR, drone and GPS survey data

PhotoSat has produced over 700 satellite surveying projects across the globe. In many of these projects, we have incorporated or used LiDAR, drone and GPS survey data previously acquired for the projects. Some-times we are asked to extend or repeat previous LiDAR drone or GPS surveys. Other times we are asked to en-sure that the PhotoSat surveys are consistent with the existing LiDAR, drone and GPS surveys and vice versa. We use some form or GPS survey data on the majority of our projects.

We have found the majority of the LiDAR survey data we have used are accurate and reliable. We have found global offsets of a few tens of centimeters in about 20% of the LiDAR data sets. This is probably due to an in-correct tie to a survey benchmark or insufficient GPS recording time when establishing a new survey bench-mark. We have found tilts in the elevation data of a few centimeters per kilometer in about 5% of the LiDAR data sets.

We have found significant elevation errors in over half of the drone data sets that have been provided by our clients. This high proportion of drone data sets with errors may be due in part to clients using PhotoSat satellite surveying to check drone surveying that they suspect may be in error.


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GPS surveys need to include the GPS re-cordings to be credible

When PhotoSat first started satellite surveying, we encountered errors in a significant proportion of GPS ground surveys. We found that many of the GPS survey points had location differences to the stereo satellite photos that were well beyond the error limits of the sat-ellite photos.

We then started requesting the raw digital GPS re-cordings along with the GPS coordinates of the survey points. With this request, the quality of the GPS survey data significantly improved. The raw digital GPS re-cordings clearly show when the GPS signals at a survey point have been recorded for sufficient time to deter-mine an accurate survey location. We have much more confidence in GPS survey data that include copies of the GPS recordings.


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Which type of surveying would be best for my project ... › wp-content › uploads › 2019 › 07 › ...Satellite surveying requires no in-country presence or permits LiDAR surveys

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