HIGH RESOLUTION DEMs AND ORTHO-IMAGE

BASEMAPS FOR LOCAL GOVERNMENT

APPLICATIONS

STATEMENT OF PROBLEM

Digital Elevation Models (DEMs) are the digital representation of topographic and/or man-

made features located on the surface of the earth. DEMs are widely used for hydrologic

analyses, resource management, transportation planning, earth sciences, and environmental

assessment. Increased use of DEMs in these wide-ranging applications has led to greater need

for higher resolution and higher accuracy digital elevation data. This is especially true for local

government applications such as basemap generation/registration, stormwater management,

watershed mapping, water-quality assessment, and flood-risk assessment because of the fine

horizontal and vertical spatial scale of most features (streets, buildings, localized watersheds,

etc.).

High resolution DEMs are also essential in producing ortho-rectified image products used as

digital basemaps in GIS-based local government applications. A digital image basemap is an

essential component in local government GIS systems because many other vector-based data

layers (streets, parcel boundaries, sewer lines, etc.) are routinely superimposed upon the image

basemap so as to provide an easily understood visual frame of reference. Digital image

basemaps must have a high degree of horizontal resolution (e.g. 1 m pixel size) and planar

accuracy so that all other data layers can be superimposed without geometric distortions.

Topographic distortions must be removed from raw airborne or satellite based digital imagery in

order to provide digital orthoimages with sufficient planar accuracy for use as digital image

basemaps.

PRESENT SITUATION

DEMs

There are several commonly used satellite data sources for high-resolution DEMs, and these fall

into two categories: optical and radar. Stereo-correlation processing of panchromatic imagery

from the SPOT satellite typically yields DEMs with a 20 m horizontal (xy) resolution. Typical

absolute RMS vertical accuracies for SPOT DEMs are on the order of 8-15. DEMs available

directly from SPOT Image, Inc. have a 20 m xy resolution and a 7-11 m RMS z accuracy.

Similarly, DEMs produced from Interferometric Synthetic Aperture Radar (IfSAR) image

processing (e.g. ERS-1/2) have yielded 25 m xy resolutions and 10-30 m RMS z accuracies.

The most widely available high-resolution DEMs in the U.S. are the 7.5-minute Level 1

DEMs available from the United States Geological Survey (USGS). These are produced from

stereo processing of 1:40,000-scale panchromatic aerial photographs which are acquired every 5-

6 years by the National Aerial Photography Program (NAPP). The USGS 7.5-minute DEMs

have a 30 m xy resolution, a 7-15 m RMS z accuracy, and are available for about 90% of the

continental U.S. The horizontal resolution and vertical accuracy of all the DEMs described

above are not suitable for the vast majority of local government applications.

Recently, this has prompted development of higher-resolution and/or higher accuracy DEM

data products from commercial remote sensing data providers. These providers use airborne

IfSAR and LIDAR (LIght Detection And Ranging) systems for topographic mapping. For

example, the Intermap STAR 3i X-band IfSAR system can provide digital elevation data with a 5

m xy resolution and a 2-3 m RMS z accuracy. Commercial LIDAR systems can provide 1.5 m

xy resolution and 0.4 m vertical accuracy. However, the current cost of commercial IfSAR and

LIDAR data is in most cases too great for widespread adoption by city and county governments

in areas other than major metropolitan cities.

OrthoImage Basemaps

The most widely available high resolution digital orthoimage dataset available to local

governments throughout the U.S. are the Digital Orthorectified Quarter Quadrangles (DOQQs)

produced by the USGS using the 30 m Level 1 DEMs in conjunction with the NAPP aerial

photography. The DOQQs have a horizontal resolution of 1 m and a planar accuracy of 10 m.

The reported 10-m planar accuracy is a circular error at 90% probability (CE90) and corresponds

to National Map Accuracy Standards (NMAS). The DOQQ horizontal resolution is sufficient for

local government basemap applications. The quoted planar accuracy is somewhat of a limitation

for widespread use. Nevertheless, many city and county governments have utilized DOQQs as

their data source for orthoimage basemaps because the cost to obtain these data is essentially

zero. However, problems with displacements between individual DOQQs in the overall basemap

and problems with proper registration of vector data layers are routinely experienced. Another

limitation of the DOQQs is that these are produced by the USGS from NAPP aerial photographs

which are only acquired about every 5-6 years. Thus, the DOQQs often are outdated in many

areas and do not reflect the current situation within the city and/or county, especially in areas that

experience rapid development.

The recent launch of the IKONOS satellite has opened up a new area for acquiring up-to-date

high-resolution panchromatic digital imagery for use as digital orthoimage basemaps. The

IKONOS panchromatic imagery has a horizontal resolution of 1 m and an 11-bit information

content. The 1 m pixel size is identical to the DOQQ horizontal resolution while the 11-bit

imagery provides image contrast and quality that is superior to the DOQQs. The lowest

precision IKONOS panchromatic image product (Carterra Geo) costs $12/km2 and has a planar

accuracy of only 50 m (CE90). While this product is affordable for most local government

entities, the planar accuracy of 50 m renders this product unusable for basemap applications.

Our interaction and discussion with basemap users within the City of Columbia and the County

of Boone indicate that a planar accuracy of 3-4 m CE90 is required for useful implementation as

a digital basemap. The highest accuracy IKONOS panchromatic image product (Carterra

Precision) costs $66/km2 and has a planar accuracy of 4 m (CE90). While this meets the planar

accuracy requirement for basemap implementation, it is 5.5 times more expensive than the Geo

product. This cost is prohibitive for most local government agencies with limited budget

resources. For example, Boone County covers an area of approximately 2,400 km2. Thus, for

complete county coverage the Geo and Precision IKONOS datasets would cost $28,000 and

$155,000, respectively. The latter figure is well beyond the budgetary capability of most local

governments. The challenge then becomes to develop a methodology that would enable the use

of the lower-cost Geo products for creation of digital basemaps.

NEW APPROACH AND POTENTIAL BENEFITS

In this research we develop and demonstrate the use of National Aerial Photography

Program (NAPP) imagery for the creation of high resolution DEMs for local government

applications. These are the same aerial photographs used by the USGS for creation of their 30 m

Level 1 DEMs and the DOQQs discussed above. However, we utilize the NAPP photos to create

significantly higher resolution DEMs than those generated by the USGS. Precision ground

control from rapid-static and kinematic GPS surveys are used to validate the vertical accuracy of

the DEMs. The results indicate that DEMs with 3-m horizontal resolution and an RMS vertical

accuracy of ~ 2 m can be generated from the NAPP data.

In addition, we create orthorectified images suitable for use as digital basemaps using the

NAPP-based DEMs and the raw NAPP photos. Furthermore, we demonstrate that the NAPP-

based DEMs can be used to orthorectify low-precision IKONOS Geo image products to generate

a suitable IKONOS-based digital basemap. A number of studies are done to demonstrate the

planar accuracy of various digital basemap datasources, e.g. NAPP, DOQQ, IKONOS, etc. The

results are then used to assess the cost/benefit of the various sources for digital basemap creation

and use for local government applications.

The benefits of this research demonstrate several important applications of remote-

sensing data products pertinent to local government needs. The data products are shown to

viable in terms of both cost and utility (resolution, accuracy, etc.). The combined results indicate

that there are significant opportunities for cost-effective generation of high-resolution DEMs and

digital orthoimage basemaps for utilization by local government entities.

TECHNICAL APPROACH

The study area chosen for this work was in Southern Boone County (SBC), Missouri just

south of the City of Columbia (population 80,000). The area is about 220 km2 (14 x 16 km) and

is part of the US 63 corridor connecting Columbia and the capital of Jefferson City. The study

area also contains the small town of Ashland (population 3,000). This is an environmentally

sensitive area containing several state parks. The area is under pressure from rapid expansion

(primarily single family housing developments) pushing south out of Columbia and north out of

Ashland.

DEMs

Ten aerial photos (B/W film positives) were obtained from the NAPP archive at a cost of

only $100 ($10/photo). The photos were acquired by the NAPP program in leaf-off conditions in

April, 1996. The photos were oriented in two N/S strips containing 5 photos/strip. The overlap

in the photos is approximately 60% N/S and 30% E/W. Stereo-coverage from the ten photos

covered an area of approximately 325 km2 (15 x 20 km). The B/W film positives were precision

scanned at 1200 dpi (0.85 m pixel size) by a third party vendor at a total cost of $200

($20/photo). Precision scanning is required to preserve the geometric integrity of the B/W

photos. A rapid-static GPS survey was conducted in the study area to obtain 35 ground control

points (GCPs) for DEM registration/generation. In addition, kinematic GPS data were collected

between GCPs to provide independent check points (ICPs) for DEM validation. The accuracy of

the rapid-static GCPs were found to be 3-5 cm RMS for all three coordinates (x, y, and z). The

vertical accuracy of the kinematic GPS data used as ICPs was found to be 10 cm RMS.

The NAPP photos were processed using commercially available software (PCI APEX

v7.0) for DEM extraction. Thus, the results generated in this study could be easily reproduced

by other parties (e.g. third-party consulting companies, GIS specialists in city/count

governments, etc.). The GCPs are used to generate a highly accurate coordinate reference frame

for triangulation and registration of the NAPP photos. DEMs with 3, 10, and 30 m horizontal

resolutions were extracted via automated stereo-correlation processing. DEMs with the different

horizontal resolutions were created to evaluate the impact of resolution on vertical accuracy and

on output products derived from this and other ICREST/Synergy I projects using the DEMs as

input (e.g. digital basemaps, hydrologic models for water quality assessment, etc.). A dataset of

1000 ICPs derived from the kinematic GPS survey was used to assess the vertical accuracy of the

DEMs. The ICP dataset was a small subset of the kinematic GPS data selected first by

eliminating low-accuracy kinematic positions (due to obstructions caused by trees and buildings

during survey). In addition, the ICP data were selected: a) for uniform distribution throughout

the study area, and b) for a minimum separation between ICPs of 30 m.

OrthoImage Basemaps

Digital orthorectified image basemaps were produced using a variety of data sources and

techniques. All orthoimage basemap products were produced using commercially available

software (PCI OrthoEngine v7.0). Thus, just as was the case for the DEM generation, these

results can be easily reproduced by other parties as needed. This is an important issue for

widespread adoption by state and local government entities. Proprietary algorithms and software

are in impediment for widespread adoption of remote-sensing information products.

First, the NAPP-based DEMs with 3 m, 10 m, and 30 m horizontal resolution were used

to orthorectify the ten raw NAPP aerial photos to produce three different NAPP digital

basemaps. In this way the effect of DEM resolution and accuracy could be evaluated in terms of

the impact on the visual quality and planar accuracy of the resulting orthoimage basemap

products. Note that these digital basemaps are produced from the same NAPP data used by the

USGS to produce the 30 m Level 1 DEMs and the DOQQs. However, the approach adopted

here should produce better results because the horizontal resolution and accuracy of the NAPP

DEMs produced in our research are significantly better than the USGS DEMs. This should then

produce better overall digital basemaps, and this assumption is validated by the results shown in

the next section.

Next, the 10 m and 30 m NAPP DEMs were used to orthorectify a low-precision

IKONOS georeferenced image. There are several options available in the orthorectification

software for establishing an accurate coordinate reference frame for the georeferenced IKONOS

data using GCP input. These options are simple polynomial (SP), rational function polynomial

(RFP), and the more rigorous sensor model (SM). The SP option was not used as this is known

to produce very poor results compared to the other two options. Both the RFP and SM options

were evaluated using the 10 and 30 m NAPP DEMs.

The visual quality and the planar accuracy of the all these digital basemaps were

evaluated. The visual quality was subjectively measured by examining the resulting linearity of

known linear features (primarily roads). The planar accuracy was assessed by using a dataset 20

ICPs whose positions were derived from the rapid-static GPS survey noted previously. The ICPs

were selected a priori at locations in the imagery that were sharp and distinct point features.

This then enables the identification of these points in the digital basemaps and subsequent

comparison with the known locations. The planar accuracy of the USGS DOQQs was also

evaluated in this manner using the same ICP dataset for comparison purposes.

Figure 1. NAPP 10 m DEM for Southern Boone County study area. Location of GCP’s indicated by red triangles. The RMS vertical accuracy of the DEM is 2.3 m.

Finally, it should be noted that another ICREST Synergy I project produced two other

digital basemaps using alternate approaches. The first product was produced for the entire area

of Boone County by combining together approximately sixty USGS DOQQs into one seamless

mosaic. The second product was produced from a low-precision IKONOS georeferenced image

covering about 40% of the county. The IKONOS digital basemap was created by performing an

image-to-image registration of the IKONOS georeferenced image to the DOQQ mosaic through

selection of several hundred tie points. The planar accuracy of these two digital basemaps was

also evaluated using a dataset of several hundred ICPs derived in a similar manner as described

above. The planar accuracy of these two products will also be shown for comparative purposes.

PRODUCTS AND RESULTS

(1) DEMs with horizontal resolutions of 3 m, 10 m, and 30 m were generated for the Southern

Boone County (SBC) study area using the ten NAPP aerial photos and the approach

described in the previous section. Figure 1 shows the NAPP DEM with a 10 m horizontal

resolution. The RMS vertical accuracies of the 3 m, 10 m, and 30 m DEMs were 3.0, 2.3,

and 2.2 m, respectively. A sample histogram of the measured vertical error using 1000 ICPs

is shown in Figure 2 for the 10 m NAPP DEM. The 3 m DEM is noisier (larger RMS error)

because stereo-correlation errors are the largest at the highest resolution. This is caused by a

reduced number of pixels used in the final iteration of the stereo-correlation matching

process. Nevertheless, the RMS vertical errors of all the DEMs are significantly better than

the quoted 7-15 m RMS vertical accuracy of the 30 m USGS Level 1 DEMs. In addition, the

3 m and 10 m horizontal resolutions are a substantial improvement relative to the 30 m

horizontal resolution USGS DEMs. This is especially important for city/county governments

that must resolve features with fine spatial detail. These DEMs were utilized as a key input

layer in another ICREST Synergy I project involving the use of hydrologic models for water

quality assessment. In addition, these DEMs were used to orthorectify the NAPP and

IKONOS images for digital basemap creation.

(2) The ten raw NAPP aerial photographs were orthorectified using the 3 m, 10 m and 30 m

NAPP DEMs to produce three seamless digital basemaps for the study area with a pixel

resolution of 1 m (all three mosaics). While the horizontal resolution of all three mosaics

was the same, the visual quality of the mosaics produced using the 10 m and 30 m NAPP

DEMs was much better than the mosaic produced using the 3 m NAPP DEM. Linear

features (e.g. streets) in the mosaic produced from the 3 m NAPP DEM were slightly warped

and distorted compared to the other two mosaics because of the larger RMS vertical error of

the 3 m NAPP DEM. An assessment of the planar accuracy of the three mosaics is presented

shortly.

(3) A low-precision IKONOS georeferenced image (Carterra Geo Product) was orthorectified

using the 10 m and 30 m NAPP DEMs to produce several different digital basemaps for the

study area. The 3 m NAPP DEM was not utilized because of the larger vertical error and its

affect on the orthoimage visual quality noted above. Digital basemaps were created for each

DEM using either the RFP or the SM software option for orthorectification. In addition, the

30 m USGS Level 1 DEM was used as well to generate digital basemaps from the IKONOS

image for comparative purposes. All digital basemaps were generated with a pixel resolution

of 1 m. Figure 3 shows the IKONOS digital basemap created using the SM option and the 10

m NAPP DEM. Comparison of the visual quality and planar accuracy of the various

IKONOS basemaps are given next.

(4) The visual quality and planar accuracy of the NAPP and IKONOS digital basemaps

generated using different DEMs and/or coordinate reference frame approach (RFP or SM for

IKONOS) were evaluated. The visual quality was assessed subjectively based upon the

ability to preserve the linearity of known linear features (e.g. streets and buildings). The

planar accuracy was computed using an ICP dataset as described in the technical approach

section. A summary of these results is given in Table I. The raw IKONOS georeferenced

image and the USGS DOQQs were also included in this evaluation for comparative purposes.

The DOQQs for our Southern Boone County (SBC) study area had a planimetric accuracy of

3.4 m (CE90). This is significantly better than that 10 m planimetric accuracy quoted by the

USGS. The county-wide seamless mosaic produced in another ICREST Synergy I project

from about three dozen DOQQs was found to have a planimetric accuracy of 3.6 m (CE90).

Thus, this is consistent with our findings for the smaller SBC study area. Our evaluation of

DOQQs from two other study areas within the State of Missouri on a NASA-funded initiative

found similar results. Thus, it appears that the planimetric accuracy of the USGS DOQQs

are often times much better than the USGS quoted accuracy of 10 m CE90, which is

probably a worst-case estimate. Our interaction and discussion with basemap users within

the City of Columbia and the County of Boone indicate that a planar accuracy of 3-4 m CE90

is required for useful implementation as a digital basemap. Thus, the DOQQs are a viable

low-cost (essentially zero) data source for digital basemap applications. The disadvantage is

that the DOQQs are often not up to date for most areas of the country. The latest DOQQs

available for our study are based on NAPP data acquired in 1996 and is therefore nearly 5-

years out-of date.

The NAPP digital basemaps have planimetric accuracies on the order of 3.1 m, which is

slightly better than the DOQQs which are based on the same NAPP aerial photos. The better

planimetric accuracy is probably due to the more accurate DEMs produced as part of this

project. While the planimetric accuracy of all three NAPP basemaps are nearly the same, the

digital basemap created using the 3 m NAPP DEM had significant distortions in the linear

image features and is therefore inferior to the basemaps generated using the 10 m and 30 m

NAPP DEMs.

The raw georeferenced IKONOS Geo image had a planimetric accuracy of nearly 24 m

(CE90) prior to orthorectification. After orthorectification, the IKONOS-based digital

basemaps had planimetric accuracies that varied between 2.7 and 4.6 m. The

orthorectification coordinate reference frames produced using the RFP method yielded the

lowest errors (2.7-3.1 m CE90). However, the RFP solution also caused a significant amount

of distortion in the linear features and is therefore inferior to the IKONOS-based basemaps

produced using the SM solution which yielded slightly worse planimetric accuracies (3.2 –

4.6 m CE90). Of the SM IKONOS basemaps, the one created using the 10 m NAPP DEM

had a much better planimetric accuracy (3.2 m CE90) compared to the basemaps created

using either the 30 m NAPP DEM (4.6 m CE90) or the 30 m USGS DEM (4.6 m CE90).

The IKONOS-based digital basemap created on another ICREST Synergy I project using the

image-to-image co-registration method and the USGS DOQQs yielded a planimetric

accuracy of 4.1 m CE90. As expected, this is slightly worse than the planimetric accuracy of

the DOQQs which was found to be about 3.5 m CE90 in two independent analyses.

Considering both the planimetric accuracy and the visual quality of the IKONOS-based

digital basemaps, we believe the best IKONOS-based digital basemap is the IKONOS – SM

created using the 10 m NAPP DEM. This yielded a very good image quality and a

planimetric accuracy of 3.2 m. The accuracy is comparable to the DOQQs but the image is

recent and of much higher quality (e.g. 11-bit). These are important and unique advantages

relative to the DOQQs.

Finally, we note that the planimetric accuracies of the IKONOS-based digital basemaps are

on the order of the 3-4 m CE90 specified by our user communities within the City of

Columbia and the County of Boone. These planimetric accuracies are achievable using the

lower-cost IKONOS Geo products in conjunction with a limited amount of GCPs (ten) and a

viable DEM. The IKONOS Geo products are 5.5 times less expensive than the highest

precision IKONOS products that have a planimetric accuracy of 4 m CE90. Thus the results

presented here demonstrate that this level of precision can be achieved using the lower-cost

IKONOS Geo product. This represents a significant opportunity to expand the market of

high-resolution IKONOS imagery for digital basemap creation for local government entities

who would otherwise be unable to afford the highest accuracy IKONOS products.

Table I. Comparison of the Visual Quality and Planar Accuracy of Various Digital Basemap

Products with 1 m Pixel Resolution

Image

Data

Source

DEM

Used

RMS

Radial

Error (m)

Circular

Error @

90% (m)

Visual

Quality

IKONOS –

Geo

N/A 19.5 23.7 Excellent

DOQQ N/A 2.0 3.4 Good

NAPP NAPP 3 m 1.9 3.0 Poor

NAPP NAPP 10 m 1.8 3.1 Good

NAPP NAPP 30 m 1.7 3.1 Good

IKONOS –

RFP

NAPP 10 m 1.8 2.9 Poor

IKONOS – SM NAPP 10 m 2.2 3.2 Very Good

IKONOS –

RFP

NAPP 30 m 1.6 3.1 Poor

IKONOS – SM NAPP 30 m 2.5 4.6 Excellent

IKONOS –

RFP

USGS 30 m 2.1 2.7 Poor

IKONOS – SM USGS 30 m 2.9 4.6 Excellent

(5) The IKONOS – SM basemap created using the 10 m NAPP DEM has been delivered to the

user communities in the City of Columbia and County of Boone. These have been

incorporated into a web-based data delivery and decision support system developed by

another ICREST Synergy I project. User evaluation of this basemap and the county-wide

DOQQ basemap created by another ICREST Synergy I project is ongoing at this time.

(6) Based upon the results summarized in Table 1, we created an orthorectified digital basemap

of a georeferenced IKONOS multispectral (MS) image. The 10 m NAPP DEM was used

along with the SM coordinate solution. Figure 4 shows the orthorectified MS basemap. The

pixel size of the MS basemap is 4 m. The planimetric accuracy of the IKONOS-MS

basemap was evaluated and found to be 5.6 m. The IKONOS-MS basemap is currently

being used by the ICREST Synergy I hydrologic modeling project to derive land-use

classifications

Figure 2. Histogram of vertical error in NAPP 10 m DEM for Southern Boone County study area. A dataset of 1000 Independent Check Points (ICPs) derived from a kinematic GPS survey was used. The RMS vertical accuracy of the DEM is 2.25 m.

0

20

40

60

80

100

120

-5 -4 -3 -2 -1 0 1 2 3 4 5

Error (m)

F r e q u e n c y

RMS Error = 2.25 m# ICPs = 1000

SUMMARY

The following summarizes the key results from this project:

1) NAPP-based DEMs can be generated with horizontal resolutions that are a ten-fold

improvement over the most widely available high resolution USGS 30 m DEMs. The ~

2-3 m RMS vertical accuracy of these DEMs are significantly better than the 7-15 m

RMS vertical accuracies quoted for the USGS DEMs.

2) The NAPP DEMs are created using commercial, off-the-shelf, software using data that is

currently available for 90% of the U.S. at very low cost ($10/photo where each photo

covers an area of about 80 km2).

3) The benefits listed in 1) and 2) demonstrate that low-cost DEMs can be generated at

resolutions and accuracies useful for many local government applications using an

existing dataset available for much of the U.S. Thus, the NAPP imagery represent a

viable low-cost data source, already in existence, that could be reasonably exploited and

utilized for many local government applications.

4) The NAPP-based orthoimages and the USGS DOQQs with 1 m horizontal resolution are

found to have planar accuracies on the order of 3-3.5 m (CE90). This is superior to the

highest precision IKONOS orthoimage product (Carterra Precision – 4 m CE90). These

therefore represent viable low-cost digital orthoproducts suitable for use as digital

basemaps in local government applications. The main disadvantage is that the NAPP

and DOQQ images are acquired in any given state only once every 5-6 years.

5) Low precision IKONOS Geo imagery can be orthorectified using commercial, off-the-

shelf software and the NAPP and/or USGS DEMs to yield high precision digital

basemaps with planar accuracies on the order of 3.2-4.6 m (CE90). This is comparable

to the highest precision IKONOS product (Carterra Precision). This effectively

demonstrates that high-planar accuracy digital basemaps can be created using the lower-

cost, lower-precision IKONOS Geo imagery. This could make the IKONOS products

affordable for many smaller local governments for digital basemap generation.

Taken altogether, the results above demonstrate the high-resolution, high accuracy DEMs

and digital basemaps can be generated at costs that are affordable for local government

applications. These represent significant opportunities for effective use of remote-sensing data

products by various user communities within city and county government entities.

Figure 3. Ikonos Panchromatic orthoimage produced from low precision georeferenced Space Imaging product using NAPP 10 m DEM. GCPs and ICPs are shown as red and yellow triangles respectively.

Figure 4. Ikonos Multi-Spectral orthoimage produced from low precision georeferenced Space Imaging product using NAPP 10 m DEM. GCPs are shown as red triangles. PROJECT PARTICIPANTS

Dr. Curt Davis, Dr. Xiangyun Wang, and Mr. Brian Baker of the Department of Electrical

Engineering carried out this project for ICREST under the Raytheon Synergy I program.