satellite data development

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There’s been much spec-ulation that satellitedata will one dayreplace traditional aeri-al photography forphotogrammetric appli-

cations. Yet even with the latest gener-ation sub-meter sensors, the conceptstill seems unlikely. Most land devel-opment activities today rely on one-foot, or even six-inch pixel orthopho-tos. But in a storm water managementapplication for the City and Countyof Denver, satellite data is being usedto dramatically reduce the time andcost associated with using orthophoto-derived data.

I recently visited Jeff Blossom, theGIS Photogrammetry Manager withthe Wastewater Management Division(WMD) of the City and County ofDenver to learn about their uniqueapplication of DigitalGlobe QuickBirdsatellite data to calculate pervious andimpervious land cover. I must admitthat the concept of charging propertyowners for the amount of runoff theycontribute to the watershed was new

to me, but apparently the practice isnot uncommon in the West. (Here inMaryland, we do not receive a sepa-rate charge for storm water runoff,although I’m sure that the cost ofmaintaining storm water facilities issomewhere in the taxes I pay.)Denver property owners receive a bi-

Satellite Data Used in LandDevelopment

>> By Marc S. Cheves, LS Top left: Kumar Navulur managesDigitalGlobe's product developmentgroup. His team developed the spectralanalysis algorithm to extract impervioussurfaces from multi-spectralimagery for this project.

Middle: Jeff Blossom, GISPhotogrammetry Manager with theWastewater Management Division of theCity and County of Denver.

Right: QuickBird satellite image usedto derive impervious polygons in a residential neighborhood of Denver.

Displayed with permission • The American Surveyor • January/February • Copyright 2005 Cheves Media • www.TheAmericanSurveyor.com

http://www.theamericansurveyor.com

monthly bill for water and sanitarysewer. The storm drainage bill comesannually. An independent survey in1998-99 showed that Denver’s stormdrainage assessments are 35 percentlower than the national average for typi-cal residential users.

In the last three years, the averageannual residential bill for storm waterrunoff has climbed from $27 to $54.The reason for the increase came as aresult of a study that was performed in2000-01 that showed that Denver needsnearly $400 million worth of improve-ments, including annual programs. Priorto the rate increases—20 percent annual-ly in 2003, 2004 and 2005—Denver wasspending $4-8 million per year, and theextra funding will allow it to spend $20-40 million annually for the next fouryears. And even though the 20 percentannual increase seems steep, the currentrate is about what it would have been ifit had been raised annually to keep pacewith inflation.

900 Miles of FacilitiesA large runoff producer, such as ElitchGardens—an amusement park in down-town Denver with an abundance ofpaved areas—pays roughly $60K annual-ly. The fee is not contributed toDenver’s general fund, but rather isused to manage and operate the runoffprogram. In addition to maintaining1,700 miles of sanitary sewers, WMDalso is responsible for 900 miles of stormdrainage facilities.

Prior to 1981, storm water had beentraditionally considered to be a non-point source of pollution, in contrast topoint sources like wastewater treatmentplants or factories. The EnvironmentalProtection Agency (EPA) eventuallydetermined that the outfall pipes ofstorm water detention facilities areindeed a point source, and the NationalPollutant Discharge Elimination System(NPDES) identified end-of-pipe dis-charges as something that could betested and monitored.

Denver has an aggressive, proactiveand award-winning program that datesback to the enactment of the CleanWater Act in the 1980s. But a quandarywas created by private facilities becauseWMD was responsible for all dischargesto state and federal waterways, whichincluded Cherry Creek and the S. PlatteRiver. The area covered is immense, andalthough GIS has been effectively imple-mented across all of the various divisions

within the government—the GIS has aseparate water quality layer—the waste-water division, with its mandate tooperate and maintain runoff facilities,was looking for ways to cut costs evenfurther.

Previously, WMD had worked with1"=660' six-inch panchromatic orthopho-tography, but because this approach isexpensive and time-consuming, it couldonly afford to re-fly every two years.The time lag between photo capture anddelivery—typically four to six months—also presented difficulties in mappingnew construction.

Enter DigitalGlobe with its 0.6 meterproduct. The QuickBird two-foot colorimagery is roughly one-tenth the cost oforthophotography and can be deliveredwithin two weeks of acquisition. Anexhaustive study was launched thatrevealed that the remotely sensed datacould indeed supplement the orthopho-tography. Because the satellite data usesthe orthophotography for rectificationpurposes, however, WMD will still useorthophotography, most likely on a bi-annual basis.

DigitalGlobe developed a custom algo-rithm based on the spectral characteristicsof pervious and impervious surfaces. Theprototype was developed within ENVIimage processing software, using theIDL custom programming language.When applied to the QuickBird imagery,the algorithm produces an impervious/pervious thematic map. This map isimported into ArcGIS to derive impervi-ous area per parcel.

The process is not perfect. Using thealgorithm to automatically classify“impervious” introduces challenges thatwould not pose a problem with manualcollection techniques. In initial tests,gravel rooftops were mis-classified asbare soil, and subsequently labeled aspervious. Tree canopies obscuringrooftops, driveways, or other imperviousareas introduce error into the automaticcollection process. Even so, with anerror rate of ±4.5 percent, the methodachieves an accuracy rate that, whencomparing the algorithm to manuallydigitizing the same QuickBird image, isstatistically very close. When manuallydigitizing the orthophotos, the accuracyapproaches ±1 percent.

Blossom explained that the mandateunder which WMD operates requiresthem to field verify everything they do.This does not mean “making measure-ments,” but rather, inspecting the

property to ensure that the classificationsare correct (that a gravel roof is really agravel roof, etc.). Manually digitizing theimpervious features for one residentialparcel takes roughly two minutes usingaerial photography, and roughly 10 min-utes using a scanned, geo-referenced siteplan. The algorithm can calculate imper-vious for hundreds of parcels in a fewminutes (roughly two parcels per sec-ond). Thus, using the algorithm presents

an enormous opportunity for increasedmapping efficiency.

Even though a visual inspection of theresults from using the algorithm versusmanual digitizing show marked differ-ences, Blossom says there is room forimprovement in the algorithm. Mostobvious is the edge fuzziness of the algo-rithm’s data. The digitized data exhibits

Above: Side-by-side comparison ofsatellite imagery and derived imperviouscover.

Right: This two-foot pixel, four-band,color QuickBird satellite image wasused to derive impervious polygons inan industrial neighborhood of Denver tocreate impervious surface maps.



no edge fuzziness, but requires interpola-tion to be used in areas where shadowsor tree canopies occlude surface features.Impervious features that lie in areas ofdark shadows cannot be collected usingthe manual technique. The algorithmcollection method was not affected byshadows, probably because the algorithmuses the near-IR band of the QuickBirdimage to classify impervious.

Another benefit lies in detectingchange. WMD’s previous method foridentifying properties that have changed

was to visually compare existing impervi-ous areas and the most recent imagery:all properties that exceed a ±40 percentdifference are re-mapped. This is atedious, time-consuming process, whichinvolves panning through large areaswhere less than one out of twenty prop-erties merit re-mapping. Using analgorithm to automate this process can

identify changed properties within min-utes for the entire city. The algorithmcompares numeric data as opposed tomanual visual comparison, eliminatinghuman interpretation errors, and produc-ing more consistent results.

Field People Use WirelessBlossom’s staff includes seven office peo-ple who perform impervious mapping.He has four field investigators whose jobis to verify assessments. They use hand-held wireless tablets, but after trying acompletely wireless on-demand setupwhich didn’t work very well, the fieldpeople now just download the portion ofthe database where they will be working.His department fields around 300 callsper month from citizens who are con-cerned about their bills, and WMD needsto be able to prove that their assessmentsare fair and accurate. Prior to 1988,WMD used the Assessor’s Office squarefootage for buildings and didn’t eveninclude driveways or sidewalks. Blossomadmitted that the automated data collec-tion system provides the most benefit innewly constructed areas. In 2000, Denverflew 6" orthos, in 2002, 1' orthos, and in2004, again flew 6" orthos. Moving for-ward, Blossom sees alternating betweenortho and QuickBird imagery on a yearlybasis. The most efficient use of QuickBirdimagery for WMD’s impervious mappingwould be to collect leaf-off images in newconstruction areas during years in whichthe city is not conducting an aerial photoflight.

With the current trend of rapid expan-sion and change occurring in the Cityand County of Denver, acquiring lowercost imagery at more frequent intervalson a citywide scale would benefit manyother city departments such asEnvironmental Health, DevelopmentEngineering Services, InfrastructurePlanning & Programming, Parks &Recreation, Transportation, Engineering,Fire, Police, Emergency Services, andCentral Planning Department. TheDenver application clearly shows thatsatellite-based imagery presents a cost-effective method of supplementingtraditional photogrammetry. With thenext generation of satellites promisingeven finer resolution, we are one stepcloser to the long-predicted, increasedutilization of satellite-based data in landdevelopment.

Marc Cheves is Editor of the magazine.

An example of an impervious surfacemap for a Denver property, which is sentto the customer with the storm bill.



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