GPS CONTROL FOR DIGITAL MAPPING 
Joseph Forrai 
Chief, Research Division and Department of Photogrammetry 
Survey of Israel 
ISPRS Commission IV 
ABSTRACT 
With the improvement of productivity and the accuracy of analytical photogrammetry, new tasks have 
arisen. Large volume of very accurate, large scale, digital maps, are demanded by architects and civil 
engineers, and the geographic and land information systems require creation of up-to-date data bases. 
Consequently, photogrammetric mapping requires accurate, rapid and economic ground control more than 
ever before. 
The GPS gives a very satisfactory answer for the increasing control demands. The author feels, that 
there are a number of coincidences between the GPS - and the photogrammetric philosophies. The analogies 
are reviewed in the paper. 
For illustrating the wide range of applications, two photogrammetric GPS control networks are presented. 
One was designed for large-scale urban mapping for engineering purposes; the other is pilot project of a 
global, homogeneous control network for medium- and small-scale mapping and remote sensing, mainly aimed 
at the establishment of the topographic data base of our National Geographic Information System. The 
results achieved are briefly analyzed and discussed. 
KEY WORDS: GPS, Ground Control, Digital Mapping, Photogrammetry. 
1. INTRODUCTION 3. PHOTOGRAMMETRY AND GPS - FIELD REQUIREMENT 
SIMILARITIES 
The increase of GPS applications is quite 
surprising. A few years ago GPS was considered a When choosing control points for photogrammetry, 
revolutionary (but rather expensive!) means for measured by any traditional method, the following 
high order geodetic network densification, and - requirements must be fulfilled: 
perhaps - for solving some difficult problems of 
different geoscience ramifications. Few of us a. Appropriate configuration from photogrammetric 
thought seriously, that within a few years, GPS point of view (control for model, strip or 
would dramatically break the bounds of higher block). 
geodesy and unhinderedly enter into the most 
usual, day-to-day surveying practice (Adler, b. Appropriate configuration from geodetic point 
1985). of view (intervisibility with other geodetic 
control points). 
The ground control for photogrammetry is one of 
the most successful GPS surveying applications. c. Well identifiable site for photogrammetric 
Let's try to give some reasons why. measurements (exact three dimensional reading 
ih in a photogrammetric instrument). 
2. PHOTOGRAMMETRY AND GPS - 
KINSHIP IN FIGURATIONS d. Suitable situation from aerial photographic 
point of view ("open sky"). 
The "central receiver" of the photogrammetric 
procedure is the aerial camera. The aerial base The simultaneous fulfillment of the above 
vector (b) is determined by the relative and postulates makes the field activity complicated 
"absolute" camera positions in two exposure by any classical way of measurements. But not by 
moments. Two ground located GPS receivers are in GPS! As no intervisibility is required between 
a similar mutual position. The so-called position ground points (paragraph 'b' can be omitted), the 
(or base) vector (d) is a result of the relative choice of the suitable site becomes incredibly 
and "absolute" position fix of the two GPS flexible including the very important requirement 
receivers. of paragraph 'c'. The requirement of the "open 
sky" stays relevant, as a trivial demand both for 
The spatial configuration is somewhat alike in photo and GPS techniques. 
both cases (Figure 1). The disparity is, that in 
photogrammetry the control points are located on 4. GEODETIC CONSIDERATIONS 
the ground and the base vector is "in the sky"; 
in GPS, inversely, the base vector is situated on After fulfilling the field requirements, what are 
the ground and the control points -the the further geodetic aspects of a: good 
satellites- are located "in the sky". In both photogrammetric control ? 
systems there is a relative motion between the 
"central receiver" and "control points". (As a. Sufficient geometry for the horizontal 
forward motion in photogrammetry, the consequent geodetic - photogrammetric control point 
doppler effect is also compensated for in GPS!) configuration. 
The theoretical ground for base vector b. Very good inner agreement for horizontal and 
determination, both in photogrammetry and GPS, is vertical photogrammetric control. 
spatial resection, demanding certain additional 
mathematical manipulations for complete solution. 
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