Vertical Accuracy Assessment of ALOS PALSAR, GMTED2010,

SRTM and Topodata Digital Elevation Models

Zuleide Alves Ferreira

1,2 a

and Pedro Cabral

NOVA Information Management School (NOVA IMS), Universidade Nova de Lisboa, Campus de Campolide,

1070-312 Lisbon, Portugal

Instituto Federal de Educação, Ciência e Tecnologia do Tocantins, Campus Palmas, Brazil

Keywords: DEM, Evaluation, Precision, Altitude, Brazilian Cartographic Accuracy Standard.

Abstract: Three-dimensional data of the Earth's surface can support several types of studies, such as hydrological,

geomorphological, environmental monitoring, among many others. But, due to the difficulty of acquiring

these data in the field, freely available Digital Elevation Models (DEM) have been widely used, and therefore,

it is increasingly necessary to check their accuracy to ensure their correct applicability according to the

appropriate scale. However, there are no studies which have assessed specifically the vertical accuracy of the

ALOS PALSAR, GMTED2010, SRTM and Topodata DEMs according to Brazilian Cartographic Accuracy

Standard (PEC). In this sense, this paper aims to evaluate the quality of the above-mentioned DEMs by using

the official high accuracy altimetric network data of the Brazilian Geodetic System. Statistical analysis of

errors results demonstrated that the DEMs have applications compatible with 1:100,000 scales or smaller than

this, and although the GMTED2010 presented a lower accuracy than the other DEMs, it also could be

classified in the same accuracy category according to the Brazilian PEC. We conclude that DEMs assessment

is very important to ensure their correct application as they can be used in many researches since these data

are available for practically all areas of the planet.

1 INTRODUCTION

Digital Elevation Model (DEM) is a generic term that

comprises both the Digital Terrain Model (DTM),

which represents the ground surface, and the Digital

Surface Model (DSM), which represents the upper

surface above the ground level, including trees,

buildings and other natural or artificial objects

(Polidori and El Hage, 2020). DEM consists of the

terrestrial surface representation supposedly free of

vegetation, buildings and other non-ground objects,

despite this term is often used in a generic way to refer

to DSM and DTM (Liu et al., 2015).

In the last years, several DEMs elaborated using

various techniques have been made freely available to

the community, thereby for better use of these

products, it is important to analyse their accuracy

aiming to identify their possible applications (Moura

et al., 2014). The assessment of DEMs quality is a

subject that requires further attention, and despite the

https://orcid.org/0000-0001-5283-5200

https://orcid.org/0000-0001-8622-6008

importance of DEMs applications in several fields,

there are no specific standardized guidelines

concerning their accuracy assessment, which

represents a challenge for this kind of geospatial

technology users (Mesa-Mingorance and Ariza-

López, 2020).

DEMs quality has been studied frequently to

assess their wide range of applications and most of

these studies consist of comparing the obtained data

from DEMs and a set of reference data generally

called control points (Polidori et al., 2014).

According to these authors, this comparison, that is

based on accuracy statistical indicators such as mean

difference, standard deviation or root mean square

error, is very important to evaluate the DEM

positional accuracy and contributes to improving the

mapping methods. Moreover, to ensure the reliability

of the data extracted from a DEM, it is necessary to

have very clear information about its coordinate

system, its cartographic projection and its datum, as

116

Ferreira, Z. and Cabral, P.

Vertical Accuracy Assessment of ALOS PALSAR, GMTED2010, SRTM and Topodata Digital Elevation Models.

DOI: 10.5220/0010404001160124

In Proceedings of the 7th International Conference on Geographical Information Systems Theory, Applications and Management (GISTAM 2021), pages 116-124

ISBN: 978-989-758-503-6

well it is necessary to consider that horizontal

positional accuracy errors can result in relevant

vertical errors in the DEM, mainly in areas of steep

slopes (Yap et al., 2019).

In Brazil, the quality of the cartographic products

is regulated by the Decree n° 89,817 published in the

year 1984, that establishes regulatory instructions for

the technical standards of national cartography.

Conforming to this decree, the cartographic products

must be classified observing the Cartographic

Accuracy Standard (Padrão de Exatidão Cartográfica

- PEC), which is a dispersion statistical indicator

relative to 90% probability and corresponds to 1.6449

times the Root Mean Square Error (RMSE). Thus,

90% of the collected points errors in the cartographic

product must present values equal to or less than those

predicted in the PEC when compared to its

coordinates surveyed in the field by a high accuracy

method (Brazil, 1984; 2016).

Many studies addressed DEMs accuracy

assessment (Hu et al., 2017; Jain et al., 2018;

Mouratidis and Ampatzidis, 2019; Varga and Bašić,

2015; Wessel et al., 2018); however, there are no

studies which assessed specifically the vertical

accuracy of the ALOS PALSAR, GMTED2010,

SRTM and Topodata DEMs according to Brazilian

Cartographic Accuracy Standard (PEC). In this sense,

this paper aims to evaluate the vertical quality of the

above-mentioned DEMs by using the official high

accuracy altimetric network data of the Brazilian

Geodetic System. Therefore, it is expected that the

obtained results from this comparison contribute to

the correct applicability of the analysed DEMs

according to an appropriate use scale in the country.

2 METHODOLOGY

2.1 Study Area

The Balsas River watershed is inserted in thirteen

municipalities and occupies an area of 12,352.5 km²,

that corresponds to about 4.5% of the total area of the

State of Tocantins (Figure 1) (Brazil, 2012). This

watershed altitudes are approximately between 200

and 800 meters considering the average sea level, that

represents more than 600 meters of altimetric

amplitude as can be seen in Figure 2. Inside Balsas

River watershed area were identified 105 stations of

the official Brazilian altimetric network which are

located along the main roads of the region (Figure 2).

Figure 1: Geographical location of the study area.

To evaluate the DEMs, we used the official high

accuracy altimetric network data of the Brazilian

Geodetic System available as orthometric altitudes.

Composed by altimetric geodesic stations implanted

along with the road network throughout the Brazilian

territory, this network was established in 1945 by

using the high accuracy geometric levelling method.

In order to ensure the integrity, consistency, and

reliability of the Geodetic Database information, level

references altitudes are recalculated periodically due

to the incorporation of new levelling lines and the

development of new data measurement and

processing techniques. According to these altimetric

data last quality assessment carried out in 2018,

87.5% of the adjusted geopotential values showed

standard deviations between 6 and 10 centimeters

(IBGE, 2019).

2.2 Data

The satellite observation program Advanced Land

Observing Satellite (ALOS) was created to support

mapping of land coverage, disaster monitoring, and

resource surveying (JAXA, 2020a). In 2006, ALOS

satellite was launched from the Tanegashima Space

Center with three sensors onboard: Panchromatic

Remote-sensing Instrument for Stereo Mapping

(PRISM), Advanced Visible and Near Infrared

Vertical Accuracy Assessment of ALOS PALSAR, GMTED2010, SRTM and Topodata Digital Elevation Models

117

Figure 2: Hypsometric maps of Balsas River watershed elaborated from the DEMs: (a) ALOS PALSAR, (b) GMTED2010,

Radiometer type 2 (AVNIR-2), and Phased Array

type L-band Synthetic Aperture Radar (PALSAR)

(JAXA, 2020a). The PRISM sensor is a panchromatic

radiometer with 2.5 meters spatial resolution at nadir

and is composed of a set of three optical systems

which produces stereoscopic images providing a high

accuracy digital surface model (JAXA 2020b).

AVNIR-2 sensor is a visible and near-infrared

radiometer aimed at mapping land use and coverage

that provides images with 10 meters spatial

resolution, and PALSAR is an active microwave

sensor capable of obtaining daytime and night-time

terrestrial observation without cloud interference

(JAXA, 2020b).

The PALSAR images acquired during the ALOS

mission were corrected geometrically and

radiometrically (Laurencelle et al., 2015). The

geometric distortions were first corrected with the use

of a DEM and, later, the radiometry adjustment was

executed in the affected foreshortening and layover

regions. After radiometric terrain correction, the

products were distributed using two resolutions.

Some products generated from high-resolution DEM

(NED13) were distributed with a 12.5 meters pixel

size, and others generated from mid ‐ resolution

DEMs (SRTM 30 m, NED1 and NED2) have a 30

meters pixel size (Laurencelle et al., 2015).

The Shuttle Radar Topography Mission (SRTM)

was executed onboard of the Space Shuttle

Endeavour during 10 days in February 2000 by using

two radar antennas to collect topographic data over

nearly 80 percent of Earth's land surface. The SRTM

international project was developed with the

partnership of the National Geospatial-Intelligence

Agency (NGA) and the National Aeronautics and

Space Administration (NASA) providing the first-

ever near-global data set of land elevations (NASA,

2020). In 2003, the SRTM data were made available

for many parts of the world with an accuracy of 3 arc-

seconds which corresponds to about 90 meters. But,

in 2014, all global SRTM data have been released

with the original measurements full resolution

equivalent to 1 arc-second, or 30 meters (NASA,

2020).

The Topodata project consists of a topographic

database elaborated through the refinement of the

GISTAM 2021 - 7th International Conference on Geographical Information Systems Theory, Applications and Management

118

SRTM data resolution from 3 arc seconds (90 meters)

to 1 arc second (30 meters) by kriging techniques

(Valeriano and Rossetti, 2012). This project was

developed to provide geomorphometric data from all

over the Brazilian territory due to the unavailability

of cartographic products in scales suitable for some

regions.

Released in 2008 and after being successively

inspected and revised, the Topodata project offers

local geomorphometric variables corresponding to

basic elements based on techniques of interpretation

and relief analysis. Thus, this project presents

variables such as slope, slope orientation, horizontal

curvature, vertical curvature and inputs for the design

of the drainage structure resulting in the generation of

an extensive database structured for free use by the

scientific community (Valeriano, 2008).

The Global Multi-resolution Terrain Elevation

Data 2010 (GMTED2010) is a digital terrain model

developed with the collaboration between the United

States Geological Survey (USGS) and the National

Geospatial-Intelligence Agency (NGA) providing

global coverage of all land areas from latitude 84°N

to 56°S for most products (Danielson and Gesch,

2011). This model is based on data derived from 11

raster elevation sources (Table 1) and it has been

generated at three different resolutions of

approximately 250, 500, and 1,000 meters, that equal

to 7.5, 15 and 30 arc-seconds, respectively (Danielson

and Gesch, 2011). Table 2 shows the original main

characteristics of each DEM evaluated in this paper.

Table 1: GMTED2010 - Input source data characteristics adapted from Danielson and Gesch (2011).

Dataset Resolution Horizontal unit

Horizontal

datum

SRTM DTED® 2 1 Arc-second WGS 84

DTED® 1 3 Arc-second WGS 84

CDED1 0.75 Arc-second NAD 83

CDED3 3 Arc-second NAD 83

15-arc-second SPOT 5 Reference3D 0.00416666 Decimal degree WGS 84

NED 0.00027777 Decimal degree NAD 83

NED – Alaska 0.00055555 Decimal degree NAD 83

GEODATA 9 second DEM version 2 0.0025 Decimal degree GDA 94

Greenland satellite radar altimeter DEM 1,000 Meter WGS 84

Antarctica satellite radar and laser altimeter DEM 1,000 Meter WGS 84

GTOPO30 0.00833333 Decimal degree WGS 84

(DTED®, Digital Terrain Elevation Data; WGS 84, World Geodetic System 1984; CDED, Canadian Digital Elevation

Data; NAD 83, North American Datum of 1983; SPOT, Satellite Pour l’Observation de la Terre; NED, National

Elevation Dataset; DEM, digital elevation model; GDA 94, Geocentric Datum of Australia 1994; GTOPO30, Global

30-Arc-Second Elevation Dataset).

Table 2: Original characteristics of the evaluated Digital Elevation Models.

DEM

Coordinate

System

Horizontal

Datum

Vertical

Reference

Spatial

Resolution

Radiometric

Resolution

ALOS

PALSAR

UTM

WGS 84 Ellipsoid*

12.5 meters

16 bits

(signed integer)

GMTED2010 Geographic WGS 84

Geoid

(EGM96)

231 meters

(7.5 arc-seconds)

16 bits

(signed integer)

SRTM Geographic WGS 84

Geoid

(EGM96)

30 meters

(1 arc-second)

16 bits

(signed integer)

Topodata Geographic WGS 84

Geoid

(EGM96)

30 meters

(1 arc-second)

32 bits

(floating point)

*The orthometric heights with EGM96 vertical datum were converted to ellipsoid heights using the ASF MapReady

tool named “geoid_adjust” (Laurencelle et al., 2015).

Vertical Accuracy Assessment of ALOS PALSAR, GMTED2010, SRTM and Topodata Digital Elevation Models

119

Figure 3: Flowchart of methodology.

Table 3: Statistical analysis of the altitude difference between control points and DEMs.

ALOS PALSAR GMTED2010 SRTM Topodata

Mean Error (m) 12.70 13.31 12.82 12.87

RMSE (m) 4.95 7.48 4.76 5.38

min (m) -3.58 -14.22 -3.21 -6.17

max (m) 22.04 39.78 20.93 23.60

Error Amplitude (m) 25.62 54.00 24.14 29.77

2.3 Methods

Figure 3 shows the flow chart of the methodology used

in this study. The first step was to download the data

from the study area, such as raster DEMs and points

official Brazilian altimetric network. To standardize

the data, it was necessary to convert the radiometric

resolution of the Tododata DEM from 32 bits (floating

point) to 16 bits (signed integer). The next step

consisted of extracting the altitudes of the ALOS

PALSAR, GMTED2010, SRTM and Topodata DEMs

at the coordinates of the reference points (official

altimetric network). But as GMTED2010, SRTM and

Topodata models were available with altitudes

referenced to the geoid (EGM96), then it was

necessary to convert the ellipsoidal altitudes of the

ALOS PALSAR DEM to orthometric altitudes (geoid)

using the MAPGEO2015 software (IBGE, 2015),

which is developed by Brazilian Institute of Geography

and Statistics in partnership with Polytechnic School of

the University of São Paulo.

Subsequently, as well as in previous studies (Jain

et al., 2018; Varga and Bašić, 2015; Wessel et al.,

2018), statistical analysis of the errors was performed,

where were calculated some accuracy statistical

indicators such as Altimetric Error (HE) (1), Mean

Error (ME) (2), and Root Mean Square Error (RMSE)

(3). We also used the Brazilian Cartographic Accuracy

Standard (PEC) to evaluate each DEM and to identify

their best application scale. It is important to highlight

that this methodology has been used in several similar

studies such as Moura et al. (2014) and Iorio et al.

(2012).

= H

REF

- H

DEM

(1)

ME

H𝑅𝐸𝐹  H𝐷𝐸𝑀

𝑛





(2)

RMSE 



∑

  









(3)

Where H

= altimetric error; H

REF

= reference point

altitude from Brazilian geodetic system official

altimetric network; H

DEM

= altitude extracted from

DEM at reference point coordinates; ME = Mean

Error; RMSE = Root Mean Square Error; and n =

number of reference points.

3 RESULTS

Table 3 shows the main results of the statistical

analysis performed in this study where it is possible to

verify that regarding the mean error, the values did not

differ much. ALOS PALSAR DEM was the one with

the lowest RMSE (4.76 m) and GMTED2010 was the

one with the worst RMSE (7.48 m). As for the

amplitude of the altimetric error, given by the differen-

ce between the minimum and maximum altimetric

errors, SRTM presented the smallest result (24.14 m)

whilst GMTED2010 presented the largest amplitude

(54.00 m). Figure 4 shows the altimetric error

distribution of each DEM where it is possible to notice

a positive distortion in all DEMs, as well as a higher

variability of errors in the GMTED2010 product.

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120

Figure 4: Histogram of the altimetric error for ALOS PALSAR (a), GMTED2010 (b), SRTM (c) and Topodata (d).

Table 4: Altimetric Cartographic Accuracy Standard of the Quoted Points and the Digital Terrain Model, Digital Elevation

Model and Digital Surface Model for Digital Cartographic Products production (Brazil, 2016).

SCALE 1:25,000 1:50,000 1:100,000 1:250,000

PEC

Class

PEC*

(m)

RMSE

(m)

PEC*

(m)

RMSE

(m)

PEC*

(m)

RMSE

(m)

PEC*

(m)

RMSE

(m)

2.70 1.67 5.50 3.33 13.70 8.33 27.00 16.67

5.00 3.33 10.00 6.66 25.00 16.66 50.00 33.33

6.00 4.00 12.00 8.00 30.00 20.00 60.00 40.00

7.50 5.00 15.00 10.00 37.50 25.00 75.00 50.00

*90% of point errors collected in the cartographic product, when compared with its coordinates surveyed in the field

by a high precision method, must present the same values or less than the predicted in this table.

Table 5: Extracted points from the DEMs (quantity and percentage) which showed altimetric errors below 15 and 25 meters.

DEM

< 15m H

< 25m

Points % Points % RSME (m)

ALOS PALSAR 71 67.6 105 100 4.95

SRTM 69 65.7 105 100 4.76

Topodata 63 60.0 105 100 5.38

GMTED2010 62 59.0 101 96.2 6.54

Table 4 presents the altimetric cartographic accuracy

standard for digital cartographic products production

and Table 5 shows the quantity and percentage of

extracted points from the DEMs which presented

altimetric errors below 15 and 25 meters.

Analysing the obtained results, it can be observed

that the DEMs assessed in this study may be included

in Class B for the 1:100,000 scale and in Class A for

the 1:250,000 scale (Table 6), since more than 90%

of the extracted points from DEMs assessed showed

altimetric errors less than 25 meters when compared

to the reference points. In this sense, these DEMs can

satisfactorily support studies that need a level of

detail compatible with scales 1:100,000 or smaller

than this considering the national cartographic

standard specifications.

Vertical Accuracy Assessment of ALOS PALSAR, GMTED2010, SRTM and Topodata Digital Elevation Models

121

Table 6: DEMs classification according to Altimetric Cartographic Accuracy Standard for Digital Cartographic Products.

Scale ALOS PALSAR GMTED2010 SRTM Topodata

1:100,000 B B B B

1:250,000 A A A A

4 DISCUSSION

This study assessed the vertical accuracy of the

ALOS PALSAR, GMTED2010, SRTM and

Topodata DEMs according to Brazilian Cartographic

Accuracy Standard aiming to contribute to the correct

applicability of the analysed DEMs in the study area

and in similar areas. It was possible to verify that all

DEMs analysed here presented satisfactory accuracy

to supply mappings in 1:100,000 scales or smaller

than this, and although the GMTED2010 presented a

lower accuracy than the other DEMs, it also could be

classified in the same accuracy category according to

the Brazilian PEC, but it should be emphasized that

studies carried out in other areas may present

different results.

Previous studies have shown that ALOS

PALSAR DEM performed better when compared to

other DEMs, such as SRTM and the Advanced

Spaceborne Thermal Emission and Reflection

Radiometer (ASTER) (Arabameri et al., 2019; Rabby

et al., 2020). However, Andrades Filho and Rossetti

(2012) stated that SRTM products have a higher

potential for delineating morpho-structural

lineaments when compared to ALOS PALSAR.

Thomas et al. (2014) also attested a relatively higher

accuracy of SRTM when compared to ASTER and

GMTED2010, where it was also found that

GMTED2010 showed the worst results due to its

larger pixel size (Thomas et al., 2015). Nonetheless,

the results presented by Mantelli et al. (2011)

demonstrated a better quality of the Topodata product

in relation to SRTM and ASTER in the

characterization of drainage networks and watershed

vectors due to its refined resolution.

Regarding the Brazilian cartographic accuracy

standard, the results presented by Moura et al. (2014)

showed compatibility with the scale of 1:50,000 for

the Topodata, SRTM, ASTER and HydroSHEDS

DEMs for watersheds with little rugged relief, but for

watersheds with higher slopes and higher drainage

density, the results showed compatibility with scales

of 1:100,000 and smaller than this.

The present study demonstrated that the evaluated

DEMs have applications compatible with 1:100,000

scales since more than 90% of the extracted points

from them showed differences less than 25 meters in

the altitudes when compared to the reference points

extracted from the high accuracy altimetric network

of the Brazilian Geodetic System. In fact, ALOS

PALSAR, SRTM and Topodata DEMs presented

100% of altimetric errors less than 25 meters and only

GMTED2010 DEM presented 3.8% of altimetric

errors higher than 25 meters.

5 CONCLUSIONS

After analysing several DEMs comparative studies,

we conclude that this kind of assessment is very

important to ensure their correct applicability

regarding the appropriate scale since these data are

available for practically all areas of the planet.

Although, there are often no precise data available for

free that can make these comparisons possible, such

as, for instance, the control points of the high

accuracy altimetric network used in this study, thus

making fieldwork indispensable.

Indeed, one of the limitations in this research was

the small number of points located within the area of

the Balsas River watershed which were not evenly

distributed as they were implanted linearly along the

banks of the Brazilian highways. But it is worth

mentioning that the availability of these data from the

Brazilian altimetric network facilitates the DEMs

assessment since it enables an accurate data analysis

without the need for fieldwork.

Particularly in this study, we found that all four

assessed DEMs can support several types of

researches provided they do not require a high level

of detail and can be represented in scales up to 1:

100,000. However, future DEMs assessments should

be based on the accuracy of a specific application,

such as hydrodynamics modelling as well as they

should investigate the correlation between altimetric

error and slope (or altitude) in the study area.

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122

ACKNOWLEDGEMENTS

This study was supported by national funds through

FCT (Fundação para a Ciência e a Tecnologia)

under the project UIDB/04152/2020 - Centro de

Investigação em Gestão de Informação (MagIC) and

the Instituto Federal de Educação, Ciência e

Tecnologia do Tocantins (Campus Palmas).

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