Digital Orthophotos

Washington County, Minnesota - Orthoimages

This page last updated: 01/14/2022Metadata created using Minnesota Geographic Metadata Guidelines

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Overview
Data Quality
Data Organization
Coordinate System
Attributes
Distribution - Get Data
Metadata Reference

Section 1: Overview

Originator:Washington County Public Works Department

Title:Washington County, Minnesota - Orthoimages

Abstract:This orthoimagery data set includes 0.5-foot digital orthoimages in 8-bit 4-band GeoTIFF and ECW tiles (20:1 compression) and an ECW county mosaic (75:1 compression). Geographic Extent: 1 county in Minnesota, covering approximately 507 total square miles. Dataset Description: The Washington County, MN 2019 Orthoimagery project called for the planning, acquisition, processing, and derivative products of imagery data to be collected at a ground sample distance (GSD) of 0.5-foot. Project specifications are based on the American Society of Photogrammetry and Remote Sensing (ASPRS) standards. The data were developed based on a horizontal projection/datum of NAD83(86) Washington County CRS, US Feet. Imagery data were delivered as 0.5-foot 8-bit 4-band GeoTIFF and ECW tiles (20:1 compression) and an ECW county mosaic (75:1 compression). Tiled deliverables contained 499 individual irregular tiles. Ground Conditions: Imagery was collected in spring 2019, while no snow was on the ground and rivers were at or below normal levels. In order to post process the imagery data to meet task order specifications and meet ASPRS horizontal accuracy guidelines, Quantum Spatial, Inc. utilized a total of 5 ground control points to assess the horizontal accuracy of the data.

Purpose:These data depict geographic features on the surface of the earth. The dataset was created to current imagery data for Minnesota.

Time Period of Content Date:

Currentness Reference:Ground condition. Image acquisition dates: May 4, 5, 6 and 7, 2019.

Progress:Complete

Maintenance and Update Frequency:Irregular

Spatial Extent of Data:Washington County, Minnesota

Bounding Coordinates:-93.0482186194
-92.7179254903
45.2985891523
44.7297171636

Place Keywords:Washington County, Minnesota

Theme Keywords:imageryBaseMapsEarthCover, 0.5-foot orthoimage, GeoTIFF, rectified photograph, rectified image, orthophoto, natural color orthophoto, orthoimage, image map

Theme Keyword Thesaurus:ISO 19115 Topic Category

Access Constraints:No restrictions apply to these data.

Use Constraints:None. However, users should be aware that temporal changes may have occurred since this data set was collected and that some parts of this data may no longer represent actual surface conditions. Users should not use this data for critical applications without a full awareness of its limitations.

Contact Person Information:John Baer, Survey Project Coordinator
Washington County Public Works Department
11660 Myeron Road
Stillwater, MN 55082
Phone: 651-430-6883
Email: john.baer@co.washington.mn.us

Browse Graphic:None available.

Associated Data Sets:For information on other aerial imagery of Minnesota, see http://www.mngeo.state.mn.us/chouse/airphoto/index.html

Section 2: Data Quality

Attribute Accuracy:Radiometry is verified by visual inspection of the digital orthophoto. Slight systematic radiometric differences may exist between adjacent orthoimage files; these are due primarily to differences in source image capture dates and sun angles along flight lines. These differences can be observed in an image's general lightness or darkness when it is compared to adjacent orthoimage file coverages. Tonal balancing may be performed over a group of images during the mosaicking process which may serve to lighten or darken adjacent images for better color tone matching.

Logical Consistency:All GeoTIFF tagged data and image file sizes are validated using commercial GIS software to ensure proper loading before being archived. This validation procedure ensures correct physical format and field values for tagged elements. Seamlines and tile edges are visually inspected. Seamline mismatches are corrected unless the overall displacement is less than one pixel.

Completeness:Orthoimages are visually inspected for completeness to ensure that no gaps or image misplacements exist within and between adjacent images. These images are derived by mosaicking multiple images to ensure complete coverage. Source imagery is cloud free.

Horizontal Positional Accuracy:The project specifications require that only horizontal accuracy be computed for orthoimagery files. The required accuracy meets or exceeds ASPRS standards. The horizontal accuracy was tested with 5 checkpoints located throughout the project area. These checkpoints were not used in the calibration or post-processing of the imagery data. The checkpoints were distributed throughout the project area and were surveyed using GPS techniques. See survey report for additional survey methodologies. The x and y positions of where the checkpoints fell on the orthoimagery were collected. These values were then compared to the surveyed control point x and y values

Vertical Positional Accuracy:There is no vertical component for orthophotos.

Lineage:Imagery Acquisition: Digital aerial imagery was obtained using a UltraCam Eagle camera equipped with Airborne GPS/IMU. A total of 13 flight lines were collected in the spring of 2019 in multi-spectral (RGB-IR) format. The 0.5-foot imagery was acquired at an altitude above mean terrain of 8540 feet to yield a pixel resolution suitable for photogrammetric mapping and orthophoto production. The imagery was collected under conditions free from clouds and cloud shadows, smoke, fog, haze, light streaks, snow, ice on water bodies, flooding, excessive soil moisture, and foliage. The imagery consisted of blue, green, red, and infrared bands. Imagery for the photogrammetric mapping and digital orthophotos was captured according to task order specifications regarding, snow, haze, and cloud cover.

Ground Control Point Collection: A total of 5 control points were established throughout the project area using a combination of conventional and GPS survey methods in order to support softcopy aerotriangulation and photogrammetric mapping meeting the accuracies specified in this Scope of Work. Ground control collection followed requirements set forth in the task order specifications. Please see the survey report for more information.

Aerotriangulation: Softcopy aerotriangulation was performed on one large block of imagery. The airborne GPS/IMU data, GPS ground control, and image coordinate measurements were utilized to allow the direct computation of the exterior orientation parameters for each image frame to support the photogrammetric process and orthophoto production.

Surface Creation: This process involved the development of seamless topographic landform elevation dataset utilizing LiDAR data from MNGeo (Minnesota Geospatial Information Office) and data from the Wisconsin State Cartographer's Office at the University of Wisconsin-Madison to support the production of digital orthophotography that meet or exceed required orthophoto horizontal accuracy. The topographic features included a grid of elevation points and may include break lines that define ridges, valleys, edge of water, transportation features and abrupt changes in elevation. The final DEM is suitable for orthophoto production only (not suitable for contour generation). The DEM is used to then generate a Triangulated Irregular Network (TIN) to support orthophoto production.

Orthophoto Processing: Utilizing all four bands [blue (B), green (G), red (R), and infrared (IR)] digital orthorectification was performed using bilinear interpolation algorithms resulting in a spatial and radiometric transformation of the digital image from line/sample space into NAD83(86) Washington County CRS, US Feet. The interior and exterior orientation parameters from the aerotriangulation process were used to project each pixel into the ground coordinate system, while the ortho grade DEM was used to correct for relief displacement. Radiometric correction software and techniques were used to create orthophoto files that minimize the appearance of image seams and without loss of feature signature. Orthophotos are checked for geometric accuracy, image quality, and are tonally balanced to produce a uniform contrast and tone across the entire project. The individual overlapping orthophoto frames were mosaicked together. The ortho photos meet ASPRS horizontal accuracy standards.

Section 3: Spatial Data Organization (not used in this metadata)

Section 4: Coordinate System

Horizontal Coordinate Scheme:Minnesota County Coordinate System NAD83

County Coordinate Zone Identifier:44

Horizontal Datum:NAD83

Horizontal Units:feet

Vertical Datum:

Vertical Units:

Depth Datum:

Depth Units:

Cell Width:0.5

Cell Height:0.5

Section 5: Attributes

Overview:Three band orthoimagery is organized in three color bands or channels which represent the red, green, and blue (RGB) portions of the spectrum. Four band orthoimagery is organized in four color bands or channels which represent the red, green, blue (RGB), and near infrared (IR) portions of the spectrum. Each image pixel is assigned a triplet or quadruplet of numeric values, one for each color band. Numeric values range from 0 to 255.

Detailed Citation:U.S. Department of the Interior, U.S. Geological Survey, 1996, Standards for Digital Orthophotos: Reston, VA.

Table Detail:

Section 6: Distribution

Publisher:Washington County, Minnesota

Publication Date:10/02/2019

Distributor's Data Set Identifier:

Distribution Liability:

Ordering Instructions:

Online Linkage:None available

Section 7: Metadata Reference

Metadata Date:01/14/2022

Metadata Standard Name:Minnesota Geographic Metadata Guidelines

Metadata Standard Version:1.2

Metadata Standard Online Linkage:http://www.mngeo.state.mn.us/committee/standards/mgmg/metadata.htm

This page last updated: 01/14/2022
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